examples: add TLS transport examples

[Rememorio]

Summary

• Add HTTPS examples for direct code clients, YAML clients, and custom TLS certificate providers.
• Add mTLS client/server examples using public testdata packages.
• Reuse existing repository test certificates instead of adding duplicate certificate files.

Tests

• go test ./features/https/... ./features/mtls/... -count=1 from examples/
• go test ./... -count=1 from examples/
• git diff --check main...HEAD
• Scanned changed files for internal-only links and local paths

[WineChord]

LGTM