Validate spec.git_provider.url

[ralphbean]

I ran into a case in the Fedora instance of Konflux where a user entered gitlab.com for their git-provider-url.

PaC accepted it no problem, but when it tried to operate on an incoming webhook, it failed hard because the url was missing its https:// prefix with errors like:

There was an error while processing the payload: Get \"gitlab.com/api/v4/projects/<project_id>/merge_requests/<merge_request_id>/discussions?page=1\": unsupported protocol scheme \"\"

and

Cannot create status: Get \"gitlab.com/api/v4/projects/<project_id>/merge_requests/<merge_request_id>/discussions?page=1\": unsupported protocol scheme \"\": Post \"gitlab.com/api/v4/projects/<project_id>/merge_requests/<merge_request_id>/notes\": unsupported protocol scheme \"\"

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PaC should start validating that field when it is submitted and reject Repository resources that don't conform to an expected schema.

Related issue for Konflux filed at konflux-ci/build-service#379.