A few cases exist where headers of optional_api provders are included but
not needed.  This causes unneeded calls to ast_optional_api_use.

* Don't include optional_api.h from sip_api.h.
* Move 'struct ast_channel_monitor' to channel.h.
* Don't include monitor.h from chan_sip.c, channel.c or features.c.

The move of struct ast_channel_monitor is needed since channel.c depends on
it.  This has no effect on users of monitor.h since channel.h is included
from monitor.h.

ASTERISK-25051 #close
Reported by: Corey Farrell

Change-Id: I53ea65a9fc9693c89f8bcfd6120649bfcfbc3478