TAPRegExt PR: per-mode limits

[Markus Demleitner]

Dear DAL, dear Apps,

In pyVO bug #685 <https://github.com/astropy/pyvo/issues/685>, we
figured it's finally time to be a bit less cavalier about the limits
that TAP services report to their clients (e.g., how long can you
take?  How large can your uploads be? etc).

In TAPRegExt PR #8, <https://github.com/ivoa-std/TAPRegExt/pull/8>, I
am proposing such a mechanism.  I'd be grateful for a review, both
technically and content-wise.

As to the latter, I am now convinced that the per-mode limits are
necessary.  But one might speculate whether we should allow even more
per-mode variation: Should we allow UDFs to only become available in
certain modes?  Or perhaps upload formats?  My current take is: no.
But if these were use cases, perhaps we need a different
architecture.

Also, the starting point of the discussion was whether services
should be allowed to say: "Please prefer async on this service" or
so.  That's not yet part of this proposal, mainly because Mark was
not convinced that's a useful thing to have.  Does anyone disagree?

And then there is the question whether the modes should be enumerated
in the schema (and thus are more or less fixed) or whether more or
other modes are conceivable (or useful).

Well: Any contributions are welcome.

Thanks,

          Markus