feat: make forge configurable in GitHub Action

Forgejo actions are a near drop-in replacement for GitHub actions. This
commit changes the action.yml file to make the forge configurable. This
allows using the action with forgejo instances.

Changes made:

- Introduce a `forge` input and pass it as the value of the `--forge`
  flag.
- Introduce `api-url` input and pass it as the value of the `--api-url`
  flag. This is required to configure the API URL of the forge. If the
  action is used with GitHub Actions it is simply ignored.
- Pass `inputs.token` as value to the `--api-token` flag. This is
  required as the forgejo client does not respect the `GITHUB_ACTION`
  environment variable.
- Introduce `owner` input and pass it as the value of the `--owner`
  flag. As this was not passed before the default is set to `""`. In
  theory it could be set to the value of `github.repository_owner`.
- Introduce `user` input and pass it as the value of the `--user`
  flag. As this was not passed before the default is set to `""`. GitHub
  Actions does not seem to provide a convenient way to get the
  repository without the owner. As such keeping it at `""` for the
  default is safest.

cmd/rp/cmd/run.go

@@ -33,7 +33,7 @@ func newRunCommand() *cobra.Command {
 		flagUsername string
 	)
 
-	var cmd = &cobra.Command{
+	cmd := &cobra.Command{
 		Use: "run",
 		RunE: func(cmd *cobra.Command, _ []string) error {
 			ctx := cmd.Context()