feat(fastrace-opentelemetry): allow custom exporter block_on

[andylokandy]

Closes #168

OpenTelemetry OTLP HTTP exporters can be configured with async HTTP clients such as reqwest::Client. Those clients may require a Tokio runtime when their export future is polled, while fastrace-opentelemetry currently drives exporter futures with a runtime-agnostic blocking executor.

This PR lets OpenTelemetryReporter callers provide the blocking strategy used to drive exporter futures, so applications can route async exporter work through the runtime required by their chosen client while preserving the existing default behavior.

[Copilot]

Pull request overview

Adds configurability to how fastrace-opentelemetry drives OpenTelemetry exporter futures to completion, enabling integration with async exporters/clients that require a specific runtime (e.g., Tokio) while keeping the existing default behavior.

Changes:

• Add OpenTelemetryReporter::with_block_on(...) to let callers supply a custom blocking/execution strategy for exporter futures (default remains pollster::block_on).
• Update docs (README + crate docs) and changelog to describe using runtime-specific blocking (Tokio example).
• Add an integration test validating that the custom block_on hook is actually invoked.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
fastrace-opentelemetry/src/lib.rs	Introduces configurable block_on strategy and wires it into export path.
fastrace-opentelemetry/tests/block_on.rs	Adds test ensuring the custom block_on callback is used.
fastrace-opentelemetry/README.md	Documents how to use async exporters by providing a runtime-specific block_on.
fastrace-opentelemetry/CHANGELOG.md	Records the new with_block_on() feature.

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