Support Authentication via mTLS device certs

Some customers use mTLS device certificates to authenticate Coder CLI and VSCode extension with Coder instead of the Access Token.

Add support for authenticating via mTLS device certs.

We support this already in our VS Code extension which was contributed by @coryb.

Start by reviewing the existing mTLS device-certificate authentication in the VS Code extension and how this macOS application authenticates the Coder CLI and VSCode extension. Clarify the relevant macOS entry points and certificate flow before implementing support. Done means both clients can authenticate with mTLS device certificates instead of an Access Token.

Written by the indexing model from the issue text.