Use this value instead of hardcoded values of 32 everywhere. This also
addresses the use of REND_DESC_ID_V2_LEN_BASE32 in
hs_lookup_last_hid_serv_request() for the HSDir encoded identity digest length
which is accurate but semantically wrong.

Fixes #23305.

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