allow extra keys in check-sources-has-meta-keys

[damiantl]

Describe the feature you'd like With the addition of different metadata in sources, it would be good to have the "--allow-extra-keys" flag also in check-source-has-meta-keys hook, similar to what we already have in check-model-has-meta-keys.

Additional context The addition of the flag would be really useful and it will have a high impact when the project has many different sources, and some of them can have more than the mandatory keys.