add IdGenerator definition to TracerProvider

[MikeGoldsmith]

Summary

Adds IdGenerator to the schema as an SDK extension plugin under TracerProvider, with a single built-in random for the spec-default random TraceId/SpanId generation.

Modeled consistently with existing SDK extension plugins (Sampler, SpanExporter, SpanProcessor). Treated as stable (no /development suffix) — IdGenerator is a stable spec extension point in trace/sdk.md, and the plugin shape (name + opaque properties bag) has nothing in flux. The property is optional and falls back to the SDK's random default when omitted.

Closes #70. Unblocks opentelemetry-js#6616 and similar work in other languages (originally surfaced from opentelemetry-cpp prototyping).

Notes

• Built-in name chosen: random (matches Java autoconfigure's OTEL_TRACES_ID_GENERATOR=random convention). The spec doesn't standardize a name.
• RandomIdGenerator has no configuration properties — the spec doesn't define any.
• Language status files have new types stubbed as unknown; the inline property-level status for id_generator inherits TracerProvider's overall status per the existing tooling behavior. Maintainers can add propertyOverrides in follow-up PRs.

Test plan

• make passes (compile-schema, validate-examples, validate-snippets, fix-language-implementations, generate-markdown)
• opentelemetry_configuration.json contains IdGenerator and RandomIdGenerator $defs
• schema-docs.md lists IdGenerator under SDK Extension Plugins
• IdGenerator_custom.yaml snippet validates against the compiled schema
• Reviewer confirms random is acceptable vs. default / default_random

[MikeGoldsmith]

ping @open-telemetry/configuration-approvers

[jack-berg]

Just a nit on the description. Thanks!

[MikeGoldsmith]

Thanks both 👍🏻