Add HLD for FRR-SONiC Protobuf Communication Channel

[cscarpitta]

The document that describes the HLD is available here: https://github.com/cscarpitta/SONiC/blob/frr-sonic-communication-channel-enhancements/doc/fpmsyncd/frr_sonic_protobuf_channel.md

[prsunny]

@cscarpitta , can you please check the following?

1. Add a section in HLD for Warmboot implications.
2. Any modifications to existing APP_DB schema with this proposal?
3. Any sequencing issues causing dependency since now some are coming via netlink and some via protobuf?

[cscarpitta]

@cscarpitta , can you please check the following?

1. Add a section in HLD for Warmboot implications.
2. Any modifications to existing APP_DB schema with this proposal?
3. Any sequencing issues causing dependency since now some are coming via netlink and some via protobuf?

@prsunny

Many thanks for the review.

1. There is no impact on warmboot.
2. No APP_DB schema modifications.
3. We reviewed this proposal in the Routing WG meeting, and I updated the proposal based on the feedback of the community. Now Protobuf is no longer used. All messages are now sent via Netlink.

[ahsalam]

@zhangyanzhao @balajib-cisco @kperumalbfn, this HLD has been discussed and approved in the SONiC Routing WG. Eddie has approved the PR. Could you please check and help to merge? Thanks!

[eddieruan-alibaba]

I could help to merge if no one has any other comments. We need this changes for Routing WG's PhoenixWing project, which upstreaming and validate SRv6 codes in 202411 release on various hardware platforms. The detail info for this project could be found at https://lists.sonicfoundation.dev/g/sonic-wg-routing/wiki/36798