chan_sip: bigger buffers for headers, better failure mode

Currently chan_sip can give weird messages if the contacts don't
fit in the From: or To: headers. This fix changes the from,to and
invite variables to use ast_str, allocates and deallocates them and
resizes them if needed.

ASTERISK-26069 #close

Change-Id: I1b68fcbddca6f6cc7d7a92fe1cb0d5430282b2b3