Add a Git repo picker form control

**BE Requirements:**
TF syntax for this parameter has not been designed and there is debate over whether it should be a distinct block or an additional syntactical layer on top of our existing blocks.

Might have to use new SDK depending on approach.
https://github.com/coder/terraform-provider-coder/pull/354

- support for GitHub and GitLab (customer hosted as well as site but we already have auth) which will require an additional API endpoint
- and support multiple repos

**FE Requirements**
- defining form type
- tests
- creating component