Tracing: fix explicit context propagation when running in javaagent

[lmolkova]

Fixes #38221

See open-telemetry/opentelemetry-java-instrumentation#11042 for the context.

TL;DR:
when azure-core-tracing-otel is running inside Java OTel or ApplicationInsights agents, the otel context is relocated.
However users passing context explicitly pass the original non-relocated context.
As a result, we can't correlate with the explicit context passed by users.

We need to handle both cases.
This change detects if we're running in the agent and converts original context to the relocated one using otel-provided public APIs using reflection.

#38221 also lists documentation, which we already have in the

azure-sdk-for-java/sdk/core/azure-core-tracing-opentelemetry/README.md

Lines 104 to 106 in 2f56b21

	Users can *optionally* pass the instance of `io.opentelemetry.context.Context` to the SDKs using key **PARENT_TRACE_CONTEXT_KEY** on the [Context][context] parameter of the calling method
	to provide explicit parent context.
	This [sample][sample_key_vault] provides an example when parent span is picked up automatically.

and

azure-sdk-for-java/sdk/core/azure-core-tracing-opentelemetry/README.md

Lines 149 to 152 in 2f56b21

	// You can pass parent explicitly using PARENT_TRACE_CONTEXT_KEY in the com.azure.core.util.Context.
	// Or, when using async clients, pass it in reactor.util.context.Context under the same key.
	String response = sampleClient.methodCall("get items",
	new Context(PARENT_TRACE_CONTEXT_KEY, traceContext));

[lmolkova]

I will add a test to validate it inside the shaded plugin

[trask]

👍