stasis: Allow filtering by formatter

stasis:  Allow filtering by formatter

A subscriber can now indicate that it only wants messages
that have formatters of a specific type.  For instance,
manager can indicate that it only wants messages that have a
"to_ami" formatter.  You can combine this with the existing
filter for message type to get only messages with specific
formatters or messages of specific types.

ASTERISK-28186

Change-Id: Ifdb7a222a73b6b56c6bb9e4ee93dc8a394a5494c