Gem-side plumbing for proxy basic auth. Port the gem side of basecamp/kamal#1865. **Pairs with:** the kamal-proxy basic-auth issue. **Verify:** `proxy.basic_auth` emits the flag; unit test.
Gem-side plumbing for proxy basic auth. Port the gem side of basecamp#1865.
Pairs with: the kamal-proxy basic-auth issue.
Verify:
proxy.basic_authemits the flag; unit test.