res_pjsip: Add per-endpoint RTP port range configuration

res_pjsip: Add per-endpoint RTP port range configuration

Add rtp_port_start and rtp_port_end options to PJSIP endpoint
configuration, allowing each endpoint to use a dedicated RTP port
range instead of the global rtp.conf setting.

This is useful for scenarios where different endpoints need isolated
port ranges, such as firewall rules per trunk, multi-tenant systems,
or network QoS policies tied to port ranges.

The implementation adds ast_rtp_instance_new_with_port_range() to the
RTP engine API, which sets the port range on the instance before the
engine allocates the transport. The default RTP engine
(res_rtp_asterisk) checks for per-instance overrides in
rtp_allocate_transport() and falls back to the global range when
none is set.

Both options must be set together, with values >= 1024 and
rtp_port_end > rtp_port_start. Setting both to 0 (the default)
preserves existing behavior.

Resolves: https://github.com/asterisk/asterisk-feature-requests/issues/71

UserNote: PJSIP endpoints now support rtp_port_start and
rtp_port_end options to configure a dedicated RTP port range per
endpoint, overriding the global rtp.conf setting.

UpgradeNote: An alembic database migration has been added to add
the rtp_port_start and rtp_port_end columns to the ps_endpoints
table. Run "alembic upgrade head" to apply the schema change.

DeveloperNote: New public API: ast_rtp_instance_new_with_port_range()
creates an RTP instance with a per-instance port range.
ast_rtp_instance_get_port_start() and ast_rtp_instance_get_port_end()
allow RTP engines to query the override. Third-party RTP engines can
use these getters to support per-instance port ranges.