In order to implement proposal 224, we need the data structure rend_data_t to
be able to accomodate versionning that is the current version of hidden
service (2) and the new version (3) and future version.

For that, we implement a series of accessors and a downcast function to get
the v2 data structure. rend_data_t becomes a top level generic place holder.

The entire rend_data_t API has been moved to hs_common.{c|h} in order to
seperate code that is shared from between HS versions and unshared code (in
rendcommon.c).

Closes #19024

Signed-off-by: David Goulet <dgoulet@torproject.org>
Signed-off-by: George Kadianakis <desnacked@riseup.net>