Add support for `schema_url`

[kaylareopelle]

The schema_url feature is not currently available for the Ruby OpenTelemetry project. Tracers, Meters, and Loggers could use the feature.

Telemetry schemas help with alignment among OpenTelemetry semantic conventions, telemetry sources, and telemetry consumers.

We could consider supporting schema_url for multiple signals at once, or just for one at a time. Note that schema_url is not optional for metrics, but is for traces and logs.

• schema_url for Meter Provider => Get a Meter
• schema_url for Tracer Provider => Get a Tracer (optional)
• schema_url for Logger Provider => Get a Logger (optional)

[github-actions]

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