allow enabling the `Thanks` extension

Also the Echo extension, as it's a dependency of Thanks. Note it's the client's responsibility to make sure all dependencies are satisfied for any new extensions added

Bug: T361521 Bug: T361522

Given a request like:

{
  "name": "test",
  "chartName": "mediawiki",
  "values": {
    "mediawikiCore": {
      "ingress": "test.catalyst-qte.wmcloud.org"
    },
    "extensions": {
      "echo": {
        "enable": true
      },
      "thanks": {
        "enable": true,
        "ref": "1020578"
      }
    }
  }
}

A MW with extensions Echo and Thanks enabled is deployed successfully:

And the commit for Thanks is the last patch set for patch "1020578" as expected. Note that the "refs/changes/25/1021325/1" format is also accepted.

The Thanks extension is functional and thanking: