Implement encoding_format injection plugin for bifrost embedding requests

[Copilot]

Some downstream LLM providers (e.g. older LiteLLM versions) require encoding_format in embedding requests, but upstream platforms like Open WebUI often omit it. This plugin intercepts embedding calls at the HTTP transport layer and injects the parameter when missing or null.

Plugin logic (main.go)

• Rewrote hello-world example into a functional HTTPTransportPreHook that detects embedding endpoints via path matching and uses gjson/sjson for zero-allocation JSON inspection and mutation of the []byte request body
• Supports float (default) and base64 encoding formats, configurable via plugin config
• Passes through non-embedding requests and requests with encoding_format already set

Configuration

plugins:
  - name: encoding-format
    enabled: true
    path: /path/to/encoding-format.so
    config:
      encoding_format: "float"  # or "base64"

CI/CD (.github/workflows/build.yml)

• Builds and tests on push/PR to main targeting Linux AMD64
• Publishes .so binary to GitHub Releases on v* tag push via softprops/action-gh-release
• Explicit least-privilege permissions scoping

Other changes

• Updated go.mod module path to github.com/lukoerfer/bifrost-encoding-format-plugin
• Updated Makefile plugin name to encoding-format
• Added 15 unit tests covering init, path matching, injection, null replacement, and skip scenarios
• Rewrote README.md with usage and configuration docs

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