Transport dialer: setting nil for wasm build

[magodo]

This PR is to make this SDK to work as an WASM module. The implementation is refered from https://github.com/golang/go/blob/071aed2aaa0ed819582c5bff44b70d43c61f504a/src/net/http/transport.go#L45.

• The purpose of this PR is explained in this or a referenced issue.
• The PR does not update generated files.
  • These files are managed by the codegen framework at Azure/autorest.go.
• Tests are included and/or updated for code changes.
• Updates to module CHANGELOG.md are included.
• MIT license headers are included in each file.

[jhendrixMSFT]

Can we get more info on what this fixes and why it's needed please?

[magodo]

@jhendrixMSFT I'm trying to build some toolings that are based on this SDK to WASM to be able to run in the browser. Say I have the following simplistic code:

func main() {
	tenantID := os.Getenv("ARM_TENANT_ID")
	clientID := os.Getenv("ARM_CLIENT_ID")
	secret := os.Getenv("ARM_CLIENT_SECRET")

	cred, err := azidentity.NewClientSecretCredential(tenantID, clientID, secret, nil)
	if err != nil {
		log.Fatal(err)
	}
	tk, err := cred.GetToken(context.TODO(), policy.TokenRequestOptions{
		Scopes: []string{"https://management.core.windows.net//.default"},
	})
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println(tk)
}

It failed with following error:

2023/08/29 09:20:38 ClientSecretCredential: unable to resolve an endpoint: server response error:
 Get "https://login.microsoftonline.com/xxxx/v2.0/.well-known/openid-configuration": dial tcp: lookup login.microsoftonline.com: Protocol not available

The reason is that the browser APIs do not allow arbitrary TCP/IP connections and therefore WASM is equally limited. The Go standard net/http library hence conditionally build the default client to mitigate this issue.

So with this change, we can then successfully send above request. Though, in order to have successfully get the token, it will need some other tricks to make it work. It is another story off this topic then.

[magodo]

Can we settle done this PR soon? As the CI sends a ton of notifications for some weired faliure like this: https://github.com/magodo/azure-sdk-for-go/actions/runs/6048886404

[jhendrixMSFT]

I was waiting for you to merge it, I'll merge it now.