res_pjsip_outbound_registration.c: Add User-Agent header override

This introduces a setting for outbound registrations to override the
global User-Agent header setting.

Resolves: #515

UserNote: PJSIP outbound registrations now support a per-registration
User-Agent header

(cherry picked from commit c7fc6ae362)

contrib/ast-db-manage/config/versions/24c12d8e9014_add_user_agent_header_to_ps_.py

@@ -0,0 +1,22 @@
+"""add user-agent-header to ps_registrations
+
+Revision ID: 24c12d8e9014
+Revises: 37a5332640e2
+Create Date: 2024-01-05 14:14:47.510917
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '24c12d8e9014'
+down_revision = '37a5332640e2'
+
+from alembic import op
+import sqlalchemy as sa
+
+
+def upgrade():
+    op.add_column('ps_registrations', sa.Column('user_agent', sa.String(255)))
+
+
+def downgrade():
+    op.drop_column('ps_registrations', 'user_agent')