Implement small service for overriding JWT secrets per EL node

[ilyaluk]

📝 Summary

This implements new tool that allows to issue JWT tokens based on multiple secrets

⛱ Motivation and Context

If we want to send CL requests to multiple ELs, and each of them has a different jwt secret, while keep using nginx for multiplexing, we need a way to dynamically generate JWT tokens for each EL. Engine API mandates that ELs should verify timestamp in JWT token to not be far off from current time[1], so we can't hardcode a JWT token in nginx config. To issue tokens dynamically we can use following options:

• Use commercial nginx for jwt module[2] (or compile OS re-implementation[3])
• Use Lua module and some libraries to work with JWT tokens[4]
• Request tokens using with auth_request module and write an external service that would issue them.

I think the latter is the optimal option considering size of changes required.

This could be implemented in sync-proxy itself, but we don't want to use sync-proxy for multiplexing yet.

This PR also based off #17 to avoid go.mod conflicts

📚 References

[1] https://github.com/ethereum/execution-apis/blob/main/src/engine/authentication.md#jwt-claims
[2] https://nginx.org/en/docs/http/ngx_http_auth_jwt_module.html
[3] https://github.com/kjdev/nginx-auth-jwt
[4] https://github.com/SkyLothar/lua-resty-jwt
[5] https://nginx.org/en/docs/http/ngx_http_auth_request_module.html

---

✅ I have run these commands

• make lint
• make test-race
• go mod tidy
• I have seen and agree to CONTRIBUTING.md