Add Git repository in pipeline as code

Goal:

As a developer, I want to add my Git repository containing pipelines in .tekton (pipeline as code) to the cluster so that the pipeline from my Git repositories is triggered and runs on the cluster based on relevant Git events.

User currently achieves this through CLI:
https://github.com/openshift-pipelines/pipelines-as-code#getting-started

Use cases:

1. User can add a Git repository to pipeline as code

Acceptance criteria:

1. User can add a Git repository to the cluster by selecting a Git repository from the list of repositories on her Git provider organisation
2. User can add public and private Git repositories
3. The Git repository is configured appropriate so that it gets triggered automatically based on relevant Git events
4. Both GitHub, GitLab and Bitbucket repositories will be supported