[confmap] Allow using any type as a string (#10800)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description

Allows any type to be used as a string if the target field is a string
or the value is expanded in inline position. Inspired by #10794. This is
not a breaking change (previously we would return an error for these).

Some things that you can do after this PR that you couldn't do before
it:

1. Set `HOST` to `[2001:db8::8a2e:370:7334]` and use it in an endpoint:
```yaml
exporters:
  otlp:
    endpoint: http://${env:HOST}/api/v1/traces
```

2. Pass really big numbers as strings (e.g. `10000000000000000000`)

3. Pass `null` as a string.

<details>
<summary>Types that can be returned by our current YAML parser</summary>

Our current way of using the YAML parser has these types: `string`,
`nil`, `int`, `uint64`, `float64`, `map[any]any`, `map[string]any`,
`[]any`.

There is no documentation for this, but the following fuzzing test did
not find any failing cases after 20 minutes of continous run:

```go
package main

import (
	"testing"
	"gopkg.in/yaml.v3"
)

func FuzzTest(f *testing.F) {
	f.Fuzz(func(t *testing.T, data []byte){
		var b any
		err := yaml.Unmarshal([]byte(data), &b)
		if err != nil {
			return
		}

		switch b.(type) {
			case string, nil, int, uint64, float64, map[any]any, map[string]any, []any:
				return
			default:
			  t.Errorf("Unexpected type %T", b)
		}
	})	
}

```
</details>

.chloggen/mx-psi_add-string-representation-for-everything.yaml

@@ -0,0 +1,26 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+component: confmap
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Allow using any YAML structure as a string when loading configuration.
+
+# One or more tracking issues or pull requests related to the change
+issues: [10800]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext: | 
+   Previous to this change, slices could not be used as strings in configuration.
+
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

confmap/expand_test.go

@@ -542,22 +542,6 @@ func TestResolverExpandUnsupportedScheme(t *testing.T) {
 	assert.EqualError(t, err, `scheme "unsupported" is not supported for uri "unsupported:VALUE"`)
 }
 
-func TestResolverExpandStringValueInvalidReturnValue(t *testing.T) {
-	provider := newFakeProvider("input", func(context.Context, string, WatcherFunc) (*Retrieved, error) {
-		return NewRetrievedFromYAML([]byte(`test: "localhost:${test:PORT}"`))
-	})
-
-	testProvider := newFakeProvider("test", func(context.Context, string, WatcherFunc) (*Retrieved, error) {
-		return NewRetrievedFromYAML([]byte("[1243]"))
-	})
-
-	resolver, err := NewResolver(ResolverSettings{URIs: []string{"input:"}, ProviderFactories: []ProviderFactory{provider, testProvider}, ConverterFactories: nil})
-	require.NoError(t, err)
-
-	_, err = resolver.Resolve(context.Background())
-	assert.EqualError(t, err, `expanding ${test:PORT}: retrieved value does not have unambiguous string representation: [1243]`)
-}
-
 func TestResolverDefaultProviderExpand(t *testing.T) {
 	provider := newFakeProvider("input", func(context.Context, string, WatcherFunc) (*Retrieved, error) {
 		return NewRetrieved(map[string]any{"foo": "${HOST}"})

confmap/internal/e2e/types_test.go

@@ -383,6 +383,16 @@ func TestStrictTypeCasting(t *testing.T) {
 			targetField: TargetFieldSlice,
 			expected:    []any{"filelog", "windowseventlog/application"},
 		},
+		{
+			value:       `[filelog,windowseventlog/application]`,
+			targetField: TargetFieldString,
+			expected:    "[filelog,windowseventlog/application]",
+		},
+		{
+			value:       `[filelog,windowseventlog/application]`,
+			targetField: TargetFieldInlineString,
+			expected:    "inline field with [filelog,windowseventlog/application] expansion",
+		},
 	}
 
 	previousValue := globalgates.StrictlyTypedInputGate.IsEnabled()

confmap/provider.go

@@ -152,7 +152,7 @@ func NewRetrievedFromYAML(yamlBytes []byte, opts ...RetrievedOption) (*Retrieved
 			val = string(yamlBytes)
 		}
 		return NewRetrieved(val, append(opts, withStringRepresentation(val))...)
-	case int, int32, int64, float32, float64, bool, map[string]any:
+	default:
 		opts = append(opts, withStringRepresentation(string(yamlBytes)))
 	}