bgpd: handle different sequence of bgp vrf create/delete

bgpd: handle different sequence of bgp vrf create/delete

BGP VRF can be created/deleted either via config or via l3vni add/del.
We need to handle various sequences.

1. If user config is presented, an l3vni del should not delete the vrf instance
2. do not write bgp config in show running for auto created vrf
2. If l3vni present, disallow the cli for deleting bgp vrf instance
3. If l3vni is added and vrf config is present set the flags properly
4. if bgp vrf is configured unset the AUTO flag

Ticket: CM-18630
Review: CCR-6906
Testing: Manual

Signed-off-by: Mitesh Kanjariya <mitesh@cumulusnetworks.com>