Add support for multiple SSH keys per Workspace so developers can authenticate to different Git repositories (e.g., personal and corporate GitHub/GitLab) without switching user profiles.

Currently, Coder only allows one key per user and platforms (GitHub/GitLab/etc) support only a single key per a repository/organization/user. Supporting multiple keys with .ssh/config (example) would align with standard local workflows and remove friction for users who want to use the same workspace to access multiple repositories with different SSH keys.