rtp_engine: Allow more than 32 dynamic payload types.

rtp_engine: Allow more than 32 dynamic payload types.

Since adding all remaining rates of Signed Linear (ASTERISK-24274) and SILK
(Gerrit 3136), only one RTP Payload Type is left in the dynamic range (96-127).
RFC 3551 section 3 allows to reassign other ranges. Consequently, when the
dynamic range is exhausted, you can go for "rtp_pt_dynamic = 35" (or 0) in
asterisk.conf. This enables the range 35-63 (or 0-63) giving room for another
29 (or 64) payload types.

ASTERISK-26311 #close

Change-Id: I7bc96ab764bc30098a178b841cbf7146f9d64964