Bazel toolchain support

[Kevintjeb]

This adds the Bazel toolchain support to change the effective Jsonnet compiler. Go-Jsonnet, Jsonnet (cpp) and Jrsonnet (Rust) are implemented.

There are some incompatibilities:

• Jsonnet (cpp) does not support the -c flag to create nested output directories.

• Jrsonnet (Rust) does not support manifest files containing the files that were generated by the Jsonnet program. It's currently only able to write those outputs to stdout.

• Jrsonnet text output (e.g. errors) is different compared to the cpp and Go implementations.

Due to these incompatibilities there are two addditional attributes on the toolchain: the flags to pass during directory creation and if manifest files are supported.

When adding rules_jsonnet as a bzlmod module, a user can now select the preferred compiler by using:

jsonnet = use_extension("@rules_jsonnet//jsonnet:extensions.bzl", "jsonnet")
jsonnet.compiler(name = "rust")

Currently there are three options: cpp, go and rust.

Additionally, this fixes CI:

The current .bazelci expects a variable ${{ jsonnet_port }} to be defined. However, that isn't the case with the current setup. The current behaviour is an empty ${{ jsonnet_port }} which causes the implementation to fall back to the default (Go), effectively disabling the cpp tests.

The CI runs:

bazel test //... --extra_toolchains=@rules_jsonnet//jsonnet:go_jsonnet_toolchain
bazel test //... --extra_toolchains=@rules_jsonnet//jsonnet:rust_jsonnet_toolchain
bazel test //... --extra_toolchains=@rules_jsonnet//jsonnet:cpp_jsonnet_toolchain

Lastly, fix docs:

The current docs setup only supports a single source file for docs generation (jsonnet.bzl). Now that toolchains.bzl is also a file that must be included in the docs generation, we use a helper file docs.bzl to aggregate what needs to be documented.

Note:

To use the Rust Jsonnet compiler a Nightly Rust version for the host tools is
required because -Z bindeps is needed to compile the Jrsonnet binary.

Add the following snippet to the Module.bazel file:

bazel_dep(name = "rules_rust", version = "0.45.1")

rust_host = use_extension("@rules_rust//rust:extensions.bzl", "rust_host_tools")
rust_host.host_tools(
    version = "nightly/2024-05-02",
)

Fixes #150

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