Merge branch 'feature/enable-git-creds' into 'master'

Enable specifying a secret for flux git creds

Add entrypoints into charts `values.yaml` for (in order):

* using an existing secret for git credentials
* create `git-credentials` from user defined `username`/`password`
* create `git-credentials` from user defined `privateKey`/`publicKey`/`knownHosts`

if none of the above are specified, repositories will assume unauthenticated access

See merge request platform-one/big-bang/umbrella!82