dirauth: Resume sending 503 directory error code

Authorities were never sending back 503 error code because by design they
should be able to always answer directory requests regardless of bandwidth
capacity.

However, that recently backfired because of a large number of requests from
unknown source using the DirPort that are _not_ getting their 503 code which
overloaded the DirPort leading to the authority to be unable to answer to its
fellow authorities.

This is not a complete solution to the problem but it will help ease off the
load on the authority side by sending back 503 codes *unless* the connection
is from a known relay or an authority.

Fixes #33029

Signed-off-by: David Goulet <dgoulet@torproject.org>

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