EPIC: Support Git Functions in buildless-variant of function operator.

[kwiatekus]

Description

Allow functions to be sourced from git repository.
Implement the new logic for git-fetching function sources with the following in mind:

• libgit2 is dead - find alternative -> GO-GIT
• allowing users to subscribe to github webhooks would improve devexp when comparing to the process of polling git repo for changes.
• We should not expect users to create git secrets in our own secret format (see our tutorial). Secrets of related k8s types (ssh key or basic auth) should be supported in serverless.
• see if we could improve performance and resource consumption for the git fetching itself - it comes into play when a function source is a small part of big monorepo and it would be a big improvement if serverless would not need to download the whole thing.

Reasons

Buildless variant of serverless should also allow for operating functions sourced from git repository.

Acceptance Criteria

• serveless, when installed in buildless-variant, can operate git functions
• libgit2 is not used in the implementation
• user can provide git credentials using secrets that follow k8s secret types for basic auth and ssh key
• users can optionally define endpoints for github webhooks (as an alternative to polling the repo)

Execution

• POC: alternative for libgit for serverless to fetch git-sourced functions #1316
• Implement basic git fetching mechanics in buildless serverless #1357
• support private registries
• support github webhooks

Testing Strategy (DoD)

Existing integration tests (incl. git-auth integration tests) should prove that git-functions work and contract of Function CRD (v1aloha2) is not broken

Attachments

Related Issues
Based on foundations developed by #1211