feat: bazel (and/or other build system) rulesets

[rbtcollins]

Most of the time Bazel users generate a ruleset outside of the project the ruleset is for. Exceptions include a) things like GRPC that are natively maintained using Bazel, and b) developer orientated products where the user interface is one of their concerns.

Pavex seems to fall into (b) - and because you've got a big focus on the developer experience, extending that out into users of Bazel, nix, pants2, buck2 and other related build systems will likely be relevant/important.

Because pavex is doing code generation, it -is- going to interact with those systems, as sqlx does (e.g. https://github.com/launchbadge/sqlx/pull/2927 ), and anything outside OSS sphere is IME increasingly likely to use such a system.

[LukeMathWalker]

This is something I'm interested in supporting as a first-party feature if there is a commercial partner who is willing to fund the work and test the integration.

Until that point, integration with non-cargo build systems will remain best-effort. Pavex works as long as there is a working installation of rustup.