extend pullrequest promotion to gitlab via ggp

[enekofb]

Close #112

Changes

• The ones specified in the ticket
• Refactored from githupr to pullrequest to we have a generic approach to any git provider. Leveraged go-git-providers for any implementation details

Tested with integration test https://github.com/weaveworks/pipeline-controller/blob/issues/112/server/strategy/pullrequest/pullrequest_integration_test.go

gitlab

github

[makkes]

Please don't forget to squash all the commits into one before merging this PR.

[enekofb]

tested e2e

{"level":"info","ts":"2022-12-08T18:27:40.920Z","logger":"promotion.handler","msg":"promoting app","app":{"apiVersion":"helm.toolkit.fluxcd.io/v2beta1","kind":"HelmRelease","name":"podinfo"},"source environment":"dev","target environment":"prod"}
{"level":"info","ts":"2022-12-08T18:27:41.790Z","msg":"committed patched manifests","strategy":"pullrequest","promotion":{"pipelineNamespace":"flux-system","pipelineName":"podinfo","environment":{"name":"prod","targets":[{"namespace":"prod"}]},"version":"6.0.2"},"commit":"e54ed0e7ea14e76f67464f6e042de6b376c37841"}
{"level":"info","ts":"2022-12-08T18:27:42.710Z","msg":"pushed promotion branch","strategy":"pullrequest","promotion":{"pipelineNamespace":"flux-system","pipelineName":"podinfo","environment":{"name":"prod","targets":[{"namespace":"prod"}]},"version":"6.0.2"}}
{"level":"info","ts":"2022-12-08T18:27:44.199Z","msg":"created PR","strategy":"pullrequest","promotion":{"pipelineNamespace":"flux-system","pipelineName":"podinfo","environment":{"name":"prod","targets":[{"namespace":"prod"}]},"version":"6.0.2"},"pr":"https://gitlab.com/e.fernandezbreis/weaveworks/-/merge_requests/1"}