feat: add rgba8 support

[spurnvoj]

Summary

• What changed:
  Adds rgba8 support to the image_encoding parameter: a new whitelist entry in GSCam::configure() and a matching "format", "RGBA" caps branch in GSCam::init_stream(). publish_stream() needed no changes since it already derives frame size/step generically via numChannels(image_encoding_).
• Why:
  Jetson's nvvidconv only natively outputs 4-byte-aligned formats (RGBA/BGRx) or YUV, never plain RGB/BGR. Without this, a pipeline ending in nvvidconv ! video/x-raw,format=RGBA had no caps gscam's appsink would accept, and failed with cannot link outelement(...) -> sink.
• Notes for reviewers:
  Same shape as add MONO16 / GRAY16_LE support #70 (MONO16/GRAY16_LE) - additive only, no changes to the generic publish path.

Impact

• Affected areas: src/gscam.cpp (configure(), init_stream()), README.md
• Breaking changes: No

Testing status

• What was tested:

  • Build/test passes
  • Tests updated if needed
  • Tested in simulation
  • Tested on real HW

• How to repeat tests:

colcon build --packages-select gscam
Verified two ways:
- On a Jetson Orin NX with an ArduCam (imx219): gscam_config ending in nvvidconv ! video/x-raw,format=RGBA, image_encoding: "rgba8" - pipeline links and publishes correctly.
- Hardware-independent repro for reviewers without Jetson hardware: gscam_config: "videotestsrc is-live=true ! video/x-raw,format=RGBA,width=64,height=64,framerate=10/1", image_encoding: "rgba8" - confirms the same caps-negotiation path without needing any Tegra-specific elements.

[clydemcqueen]

LGTM