feat: github_host option defaulting to github.com

[jsirianni]

Github Enterprise users may have an alternative URL that they connect to when cloning repos. This URL will contain the required authentication user / token.

Failures look like this when set to a bad host: https://github.com/jsirianni/bindplane-gitops-example/actions/runs/8634598214/job/23670409872

remote: Invalid username or password.
fatal: Authentication failed for 'https://github.com/jsirianni/bindplane-gitops-example.git/'

When token and github_url are unset, write back correctly fails with: https://github.com/jsirianni/bindplane-gitops-example/actions/runs/8634613162/job/23670463116

Validating options and configuring client profile.
token or github_url are required when target_branch is set.

Leaving the option unset defaults to using github_token and passes: https://github.com/jsirianni/bindplane-gitops-example/actions/runs/8634561203/job/23670291925