Add fields to get branch protection API to match GitHub APIs

[annadorottya]

Upon comparing the API response of the get branch protection endpoint and the struct implemented here I noticed a few fields missing:

• required_pull_request_reviews.url string
• required_signatures.enabled boolean
• required_signature.url string
• required_status_checks.contexts []string
• required_status_checks.contexts_url string
• required_status_checks.url string
• url string

Not sure if the url types of fields are left out on purpose but there are some cases when they are implemented (eg. enforce_admins.url) so I included them.

[gmlewis]

Thank you, @annadorottya . Will you be providing a PR, or do you want me to open up this issue to other contributors to this repo?