res_pjsip: Use hash for contact object identity instead of Contact URI.

In the wild it is possible for Contact URIs to be quite long as
parameters can exist on them. This can present a problem when storing
them in the AstDB as the URI is used as part of the object name and
there is a fixed length limit for the AstDB. This will cause
the contact to not get stored.

This change uses the MD5 hash of the Contact URI as part of the
object name instead. This has a fixed length which is guaranteed
to not exceed the AstDB length limit.

ASTERISK-25295 #close

Change-Id: Ie8252a75331ca00b41b9f308f42cc1fbdf701a02