[swss] add static anycast gateway support

[yanjundeng]

What I did Resync the original SAG support to latest master branch: https://github.com/sonic-net/sonic-swss/pull/1974

• Add SAG support in intfmgr and intforch
• Add support for SAG mac-address related IPv6 link-local toMe route
• Add related testcases

Some improvements based on the original PR:

• only add SAG-related IPv6 linklocal route for the VRF which has SAG-enabled interface
• add SAG related FDB entry into linux bridge for the SAG-enabled vlan interface
• Recover the vlan interface's mac in linux kernel to system mac once this SAG is disabled internally, for example unbind VRF or delete VRF case
• fix some pytest issues in pipeline

Why I did it

• To support static anycast gateway feature

How I verified it

Details if related