[bgpcfgd] Remove unnecessary dependency for StaticRouteMgr

[shi-su]

Why I did it

Static route configuration should not depend on BGP_ASN. Remove the dependency on BGP_ASN for StaticRouteMgr.
Fix #8027

How I did it

Check if BGP_ASN field before configuring static route redistribution and wait until BGP_ASN is available to enable static route redistribution.

How to verify it

Add unit test to cover the scenario and verify the functionality on a virtual switch.

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106

Description for the changelog

A picture of a cute animal (not mandatory but encouraged)

[liat-grozovik]

@vivekreddynv could you please review and provide feedback that indeed it is fixed following the issue you raised?
@shi-su i suggest to have it in 202012 and 202106. Please check it can be taken to these branches and set proper label.

[vivekrnv]

Verified.

admin@sonic:~$ sudo config route add prefix 20.0.0.0/24 nexthop 30.0.0.2
admin@sonic:~$ show ip route 20.0.0.0/24
Routing entry for 20.0.0.0/24
  Known via "static", distance 1, metric 0, best
  Last update 00:00:08 ago
    30.0.0.2 (recursive), weight 1
       10.210.24.1, via eth0, weight 1

admin@sonic:~$ show logging -f | grep bgpcfgd
Jul  6 06:03:15.525457 sonic DEBUG bgp#bgpcfgd: Received message : '('20.0.0.0/24', 'SET', (('blackhole', 'false'), ('distance', '0'), ('ifname', ''), ('nexthop', '30.0.0.2'), ('nexthop-vrf', '')))'
Jul  6 06:03:15.525550 sonic DEBUG bgp#bgpcfgd: Static route 20.0.0.0/24 is scheduled for updates
Jul  6 06:03:15.525887 sonic DEBUG bgp#bgpcfgd: execute command '['vtysh', '-f', '/tmp/tmp_7p8s_gz']'.