option to specify extensions with github-cli feature

[tonynajjar]

I'd like to install github-cli with e.g, the copilot extension

[samruddhikhandale]

Currently, we don't have a way where a Feature option can control the addition of extensions. We have to hardcode and add it in https://github.com/devcontainers/features/blob/main/src/github-cli/devcontainer-feature.json

Said that, not sure how many folks would appreciate the automatic installation of copilot extension. Let's keep this issue open to gather more feedback from the community. Thanks!