Refactor bootstrap to make use of go-git-providers

[hiddeco]

This pull request changes the implementation used to connect to Git
providers like GitHub from fluxcd/pkg/git to fluxcd/go-git-providers,
eliminating quite some bugs that were present in the GitLab integration.

In addition to this, the bootstrap logic has been factored out into an
(for now) internal package with two bootstrap "runners" PlainGitBootstrapper
and GitProviderBootstrapper. 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.

By doing this, it was possible to introduce a new flux bootstrap git
command that bootstraps Flux to a plain Git repository and applies the
resources to the cluster, prompting the user to upsert the public key
by hand if a private key was generated.

As a finishing touch, almost all options are now configurable like they
already were for the flux <install|create> commands, making it possible
to have much finer grain configurations like e.g. the Git author that
makes the commit, the type of key that is generated, adding a [skip ci]
appendix to the commit message, and many more.

Future enhancement possibilities:

• Assign different rights than maintainer to GitHub and GitLab teams
• Provide another Git implementation so bootstrapping to e.g. Azure DevOps is possible
• ...

Help testing this PR 🦸

1. Clone the repository and checkout the branch:

   $ git clone https:/fluxcd/flux2.git
   $ cd flux2
   $ git checkout -b go-git-providers-bootstrap origin/go-git-providers-bootstrap

2. Build the binary (requires Go 1.16 and Kustomize):

   $ make build

3. Run one of the bootstrap commands:

   $ ./bin/flux bootstrap git --help
   $ ./bin/flux bootstrap gitlab --help
   $ ./bin/flux bootstrap github --help

Ref list

Fixes #799
Fixes #835
Fixes #916
Fixes #924
Fixes #1021
Fixes #1129
Fixes #1143

Addresses part of #43

[somtochiama]

The --owner flag description should highlight that it only accepts the actual path / textual path for GitLab groups.

Great work @hiddeco 😁. It has been working okay so far.

[hiddeco]

From @somtochiama's testing:

$ flux bootstrap gitlab --owner=root --repository=fleet-infra --branch=master --path=./staging-cluster/flux-system --hostname=https://custom --personal

returns

✔ repository "https://https://example.cm/root/fleet-infra" reconciled
► cloning branch "master" from Git repository "https://https://example.com/root/fleet-infra.git"
✗ failed to clone repository: Get "https://https://example.com/root/fleet-infra.git/info/refs?service=git-upload-pack": dial tcp: lookup https on 192.168.0.1:53: no such host

but running without https in --hostname:

► connecting to example.com
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x23a10e8]
goroutine 1 [running]:
github.com/fluxcd/go-git-providers/gitprovider/cache.(*cacheRoundtripper).RoundTrip(0xc000428120, 0xc00094c200, 0xc000428120, 0x0, 0x0)
	../go/pkg/mod/github.com/fluxcd/[email protected]/gitprovider/cache/httpcache.go:71 +0xc8
net/http.send(0xc00094c200, 0x2c3d680, 0xc000428120, 0x0, 0x0, 0x0, 0xc000428af8, 0x203000, 0x1, 0x0)
	/usr/local/go/src/net/http/client.go:251 +0x454
net/http.(*Client).send(0xc0008b4030, 0xc00094c200, 0x0, 0x0, 0

Likely related to fluxcd/go-git-providers#55

[stefanprodan]

LGTM

Awesome work @hiddeco 🥇 🎖️ 🏅