feat(header forwarding): add HeaderForwarder and plumbing

[potterbm-cb]

Motivation

In order to support some common use cases, Mesh servers need the ability to pass request headers through to the native nodes, and to return headers from native node requests back to the caller.

Use cases:

• passing a client ID header to the nodes — this client ID shouldn't originate with the Mesh server since it may have many different callers, so it should be passed through to the native node requests
• sticky session cookies — to support sticky sessions on native nodes, AWS uses cookies. Cookies are managed by the Cookie and Set-Cookie headers, which will need to be passed from the caller to the native node, and returned from the native node ALB to the caller

Solution

The HeaderForwarder type extracts headers from potentially many native node requests and adds them to a Mesh response. HeaderForwarder can be injected into native node network requests as an http.RoundTripper, and can be applied to Mesh servers as middleware

A few key features:

• implements http.RoundTripper so that it can be used to create an http.Client or other client
• implements a common middleware pattern: func(http.Handler) http.Handler
• assigns every request a unique ID (which is used to associate headers from native node responses back to Mesh request/responses)
• allows forwarding only specific headers

Open questions

1. Currently there is no proof-of-concept for non-geth chains, so it remains to be seen if this architecture will work well for cosmos chains or other.

[cb-heimdall]

✅ Heimdall Review Status

Requirement	Status	More Info
Reviews	✅ 3/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2

[marklandgrebe-cb]

What if a sticky session cookie is provided in the request but it expires and a new sticky session cookie comes in the response?

[potterbm-cb]

technically since the Cookie and Set-Cookie header are different, headers for requests with cookies will always be forwarded.

If that's eventually not what we want we'd have to write custom logic to associate Cookie and Set-Cookie together for request/response (and storing/accessing) respectively

[jingweicb]

Hi @potterbm-cb , some CI checks failed, can you try to fix them, same for the other pr.

[marklandgrebe-cb]

lgtm, we should work on covering this repo with unit tests

[potterbm-cb]

closed and reopened PR to try to get the CI unstuck ("Minimum required reviews" wasn't registering the reviews)