Adapt solver-service for ocaml-ci and ocaml-docs-CI

[moyodiallo]

This is the steps.

• [x] Upgrading solver-service to ocaml-ci one.
  • the solver in solver-service is from ocaml-ci solver at some point. The last code of ocaml-ci's solver don't change much.
• [x] Adapt the protocole of solver-service for more opam-repository.
• [x] Split schema.capnp file to 2 two file solve.capnp and build.capnp
• [x] Connect ocaml-multicore-ci using the protocol from solver-service-api library (vendored). https://github.com/ocurrent/ocaml-multicore-ci/pull/37
• [x] Connect ocaml-ci to submit solves https://github.com/ocurrent/ocaml-ci/pull/634

[tmcgilchrist]

• [ ] Application and Operational Metrics for solver as a service https://github.com/ocurrent/solver-service/issues/57
• [ ] Support ocaml-docs-ci solving requests https://github.com/ocurrent/ocaml-docs-ci/issues/86
• [x] Return current solver machine (toxis) to general ocluster usage https://github.com/tarides/infrastructure/issues/29 Toxis is dead!
• [x] Deploy solver-worker to linux-arm64 machine
• [x] Extend solver to support multiple opam-repository checkouts https://github.com/ocurrent/solver-service/issues/53 https://github.com/ocurrent/solver-service/pull/71
• [ ] Extend solver requests to include caching hints, similar to build jobs, so that repeated solves re-use the same machine. Implies scaling solver pool to > 1 machine
• [ ] Investigate ways of splitting and canonicalising dependencies to improve sharing of cached results See patrick's comment.