Add API compatibility linting to CI

[zeitlinger]

Summary

Add automated API compatibility checking to CI to detect source-incompatible
and binary-incompatible changes before they are released.

Motivation

In 1.5.0, PR #1800 changed MetricWithFixedMetadata.getMetadata() from
protected to public — a source-incompatible change that was not caught
during review (see #1897).

Tools to add

Both are used by OpenTelemetry Java
and complement each other:

1. japicmp — Maven plugin that
   compares two versions of a JAR and reports binary/source incompatibilities.
   Catches changes like visibility narrowing, method removal, signature
   changes, etc.
2. Animal Sniffer — checks
   that code only uses APIs from a defined API signature. Useful for enforcing
   a stable public API baseline.

Acceptance criteria

• japicmp integrated into CI, failing on binary/source-incompatible
  changes to public/protected API
• Animal Sniffer integrated into CI, checking against a defined API
  signature baseline
• Mechanism to explicitly allow intentional breaking changes (e.g.,
  baseline file or annotation)
• Documented in CLAUDE.md / contributor guide