Adding HLD for OC Yang interfaces.

[nagarwal03]

What we did:
Added transformer support for OpenConfig Interfaces.

Why we did it?
Earlier, intf_app.go was used for interfaces configuration. Replacing it with xfmr_intf.go to add transformer support for OpenConfig YANG interfaces configuration via REST and gNMI.

Support added:

1. Provide support for OpenConfig Interfaces YANG models.
2. Replace translib App based implementation with a Transformer based implementation of:
   • Get Ethernet interface attributes.
   • Configure/ Set Ethernet interface attributes.
   • Delete Ethernet interface attributes.
3. Ethernet interface parameters can be modified via REST and gNMI.
4. Support port-speed and auto-negotiation on Ethernet interfaces via REST and gNMI.
5. Support IPv4 and IPv6 address configuration on Ethernet interfaces via REST and gNMI.
6. This does not cover gNMI subscription. This is planned for the next phase.

Corresponding coding PR:
sonic-mgmt-common changes

[linux-foundation-easycla]

• ✅login: nagarwal03 / (c50c983)
• ✅login: nagarwal03 / (c50c983, 42b465c)

The committers listed above are authorized under a signed CLA.

[adyeung]

@sayedsaquibbrcm will help review

[sayedsaquibbrcm]

Why are we not supporting Vlan interfaces?

[bhavini-gada]

Why are we not supporting Vlan interfaces?

First interface type being supported is physical.