bootstrap.go

This commit factors out the bootstrap logic into a new `bootstrap`
package, while also moving to `go-git-providers` to handle things
around Git providers (e.g. repository creation, deploy key
upsertions).

The `GitProviderBootstrapper` is a superset of the
`PlainGitBootstrapper` that besides `Reconciler` also implements the
`RepositoryReconciler`.

The Git actions rely on an interface, making it easier to support
other implementations than `go-git` at a later moment, to for example
support bootstrapping to Git servers that only support the v2 protocol.

Signed-off-by: Hidde Beydals <hello@hidde.co>