Add OTEP for OTLP exporter fallback endpoint support

[MJarmo]

Fixes #4973

Changes

This PR proposes a new OTEP for optional fallback endpoint support on OTLP exporters.

When configured, an OTLP exporter sends to the primary endpoint first (with the existing retry policy). If export fails after retries are exhausted due to a transient error, the exporter attempts the same export to a fallback endpoint (with the same retry policy). If no fallback is configured, behavior is unchanged.

Proposed configuration:

• OTEL_EXPORTER_OTLP_FALLBACK_ENDPOINT — generic fallback for all signals
• OTEL_EXPORTER_OTLP_<SIGNAL>_FALLBACK_ENDPOINT — signal-specific override (traces, metrics, logs)
• Programmatic setFallbackEndpoint(String) on OTLP exporter builders

V1 scope (intentionally narrow):

• SDK OTLP exporters only (Collector out of scope)
• Single fallback endpoint
• Failover only on transient errors after primary retries are exhausted
• Fallback shares protocol, TLS, headers, timeout, and compression with primary
• Each export attempt tries primary first (no sticky failover)

This is an OTEP proposal only. Spec integration (specification/protocol/exporter.md), declarative configuration schema updates, and language implementations are follow-up work after OTEP approval.

For non-trivial changes, follow the change proposal process.

• Related issues Proposal: Fallback/failover endpoint support for OTLP exporters #4973
• Related OTEP(s) # (this PR is the OTEP; ID will be assigned from the PR number on merge)
• Links to the prototypes (when adding or changing features)
  • Java proof-of-concept: Add fallback endpoint support for OTLP exporters opentelemetry-java#8197
  • Go proof-of-concept: Add OTLP fallback endpoint support to exporters apeirora/opentelemetry-go#7
• CHANGELOG.md file updated for non-trivial changes
  • Not applicable — this PR adds an OTEP only; CHANGELOG will be updated in the follow-up spec integration PR
• Spec compliance matrix updated if necessary
  • Not applicable — no specification requirements changed yet
• Declarative config data model is updated if SDK config surface is changed
  • Not applicable in this PR — opentelemetry-configuration schema update is planned for the spec integration PR