This change includes the following:
* New `messageVersion` field to the remote write configuration to select
between v1 and v2 message formats.
* New `remoteWriteReceiverMessageVersions` to select which message
format(s) to accept when the remote receiver is configured.
* Stricter validation for the remote write's URL (not breaking since an
empty URL value would already fail the Prometheus configuration
loading)
Signed-off-by: Simon Pasquier <spasquie@redhat.com>