[EVPN MH] 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

• Add l2_ecmp_group to objectTypeGetAvailability

Depends on

• https://github.com/sonic-net/sonic-swss/pull/3226
• https://github.com/sonic-net/sonic-swss-common/pull/890
• https://github.com/opencomputeproject/SAI/pull/2042
• https://github.com/sonic-net/sonic-buildimage/pull/19491
• https://github.com/sonic-net/sonic-utilities/pull/3405
• https://github.com/sonic-net/sonic-utilities/pull/3409