[EVPN MH][SAI] add support for EVPN MH feature

[RuslanValovyiGL]

Why I did it

This is a part of the implementation of EVPN MH feature according to requirements: https://github.com/sonic-net/SONiC/pull/1702

How I did it Support SAI API for L2_ECMP_GROUP and non-DF filtering

This change includes:

• new objects: L2_ECMP_GROUP and L2_ECMP_GROUP_MEMBER
• new API for L2_ECMP_GROUP
• new bridge port type L2_ECMP_GROUP
• new bridge port attr L2_ECMP_GROUP_ID
• new bridge port attr NON_DF for filtering BUM in EVPN MH setups