Fix: Skip agent authorization check for environment creation in REST API

This change fixes a regression introduced in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/182221
which broke the cluster agent bootstrapping workflow. When creating environments via the REST API,
we now skip checking for cluster agent authorization using a new `skip_agent_auth` parameter.

During bootstrapping, environments are created before the agent connects
for the first time, which means there are no DB entries for agent
authorization yet. The previous behavior caused a 400 Unauthorized error,
breaking glab cluster agent bootstrap commands.

Related to https://gitlab.com/gitlab-org/cli/-/issues/7786.

Changelog: fixed