stateful_session: Add Header based session state

[Pawan-Bishnoi]

trafficstars

Commit Message: "stateful_session: Add Header based session state"

Additional Description: Envoy already supports cookie based state management for sticky sessions. This PR adds one more option - header based state management (as discussed here). The header name is configurable via API. The responsibility to pass the said header with every request falls on the client. ~~The rest of the behaviour is consistent with the cookie based approach.~~ Path match is only available via StatefulSessionPerRoute.

(Reviewers may refer the existing cookie based implementation while reviewing this)

Risk Level: Low Testing: Unit & Integration tests added. Docs Changes: Added. Release Notes: Waiting. Platform Specific Features: N/A Fixes #22475

API Considerations:

1. Looking for guidance on this point. I have kept the new proto files in this repo to keep them consistent with the cookie based approach.
2. Could not find any guidance about updating the codeowners file, please see if that needs to be corrected.

[repokitteh-read-only[bot]]

Hi @Pawan-Bishnoi, welcome and thank you for your contribution.

We will try to review your Pull Request as quickly as possible.

In the meantime, please take a look at the contribution guidelines if you have not done so already.

:cat:

Caused by: https://github.com/envoyproxy/envoy/pull/23145 was opened by Pawan-Bishnoi.

see: more, trace.

[repokitteh-read-only[bot]]

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/). envoyproxy/api-shepherds assignee is @lizan CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

:cat:

Caused by: https://github.com/envoyproxy/envoy/pull/23145 was opened by Pawan-Bishnoi.

see: more, trace.

[Pawan-Bishnoi]

done @lizan Ptal.

[Pawan-Bishnoi]

@wbpcode ptal.

[wbpcode]

/wait-any