Add support for specify rev in place of tag in rosdistro_additional_recipes.yaml

[traversaro]

In RoboStack/ros-humble#320 the additional recipes required by the emscripten build were using commits as reference instead of tag, so for example using:

rmw_wasm_cpp:
  tag: a8e824af1c9d8683bc5ba5b6cbd210ddd8947dc9
  url: https://github.com/ros2wasm/rmw_wasm.git
  version: 0.0.2
  additional_folder: rmw_wasm_cpp

resulted in the rattler-build error:

 ╰─────────────────── (took 0 seconds)
Error:   × Failed to run git command: failed to git fetch refs from origin: fatal:
  │ couldn't find remote ref refs/tags/
  │ a8e824af1c9d8683bc5ba5b6cbd210ddd8947dc9
  │ 

this PR permits to explicitly use rev in place of tag in ``, for example:

rmw_wasm_cpp:
  rev: a8e824af1c9d8683bc5ba5b6cbd210ddd8947dc9
  url: https://github.com/ros2wasm/rmw_wasm.git
  version: 0.0.2
  additional_folder: rmw_wasm_cpp

to permit to specify checkouts in place of tags.