res_pjsip: Add CLI "pjsip dump endpt [details]"

Dump the res_pjsip endpt internals.

In non-developer mode we will not document or make easily accessible the
"details" option even though it is still available.  The user has to know
it exists to use it.  Presumably they would also be aware of the potential
crash warning below.

Warning: PJPROJECT documents that the function used by this CLI command
may cause a crash when asking for details because it tries to access all
active memory pools.

Change-Id: If2d98a3641c9873364d1daaad971376311aef3cb