[UI/BE] Allow Git Repository selection for Git Authentication

[AlinaGoaga]

Closes #1879

What changed?

• GitAuth section displays a dropdown of git repos that the user has access to and allows the user to select the git repo to be used when the PR is created (this impacts create/delete cluster and create app)
• If the git repo the user chooses is in ssh format, we convert it to https before adding it to the PR payload
• For delete/edit cluster, we use the git repo url where the creation PR was added and disable the dropdown (for links that were initially ssh we convert them from https back to ssh to display)
• If a git repo is annotated with ['weave.works/repo-rule'] : default it should appear as default in the dropdown; if this isn't present, the default will be the flux-system/flux-system gitrepo
• BE was updated to use the repository Url sent in the create PR payload (it was before ignoring this and only using config values)
• Cluster table has been updated to allow the selection of only one cluster as this is used solely for Delete Clusters which for the time being will only handle one cluster (handling multiple clusters would have been problematic since we can have different git repos where their PRs were initially created)

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