Allow HDU subclasses to override order in which match_header is called

[astrofrog]

This PR adds the ability to define __match_header_priority__ on HDU sub-classes to override the order in which HDU classes are tested against a header with match_header. This is in order to simplify the implementation of #15474

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[astrofrog]

Note quite ready yet as a lot of test failures!

[astrofrog]

Should be ready now

[astrofrog]

This doesn't quite solve the issue in #15474 (comment) so we can hold off for now - marking as draft.