extsvc: Replace raw strings with constants for external service "kind"
Created by: ryanslade
This change replaces all uppercase occurrences and places where we were doing a switch on 'string.ToLower(kind)`
I've not made changes to occurrences of lowercase strings yet apart from where they were obviously referring to 'kind' as we also use those to represent repo.ServiceType
and I'm not clear on how they interact yet.
For example: https://github.com/sourcegraph/sourcegraph/blob/8bea5eb42c72423a163932d6f31d9b5cf9ebb21d/internal/api/api.go#L48-L51
Part of: https://github.com/sourcegraph/sourcegraph/issues/10296