Add custom tag to go_sdk

[ylecornec]

Depends on #3543

What type of PR is this?

Feature

What does this PR do? Why is it needed?

This PR adds a new custom tag to the go_sdk module extension.
This tag is used to declare custom go toolchains to rules_go, to take them into consideration when creating the go_host_compatible_sdk repository introduced in #3543.

Which issues(s) does this PR fix?
See bazel-contrib/bazel-gazelle#1469

Other notes for review

• The attribute of the custom tag is not directly the label of a toolchain, but the label of a file containing a toolchain variable. This way the file can be generated by the user in a repository rule (and the toolchain variable can be set to None to be deactivated if some conditions are not met).

• Custom toolchains have precedence over other ones (defined in the same module) because the user can deactivate them if needed.

[fmeum]

Thanks for performing the split.

This adds sufficient complexity that I would first want to get back to the formulating the exact use case(s) we are trying to address with it. Non-Bzlmod rules_go offers go_wrap_sdk, which I think may help us here as well.

@ylecornec If we had e.g. go_sdk.wrap(root_files = {"linux_amd64": "@my_nix_go_sdk//:ROOT"}), would that adress your use case? If not, could you provide more details including perhaps some code snippets to explain how you fetch your SDKs?

[ylecornec]

Ah, something like go_sdk.wrap is close, but to replicate the setup of non bzlmod rules_haskell, the @my_nix_go_sdk//:ROOT value would need to depend on some environment variable (to test whether we are in a nix-shell or not).

Here is an example use-case that replicates this with bzlmod (using the custom tag) and has more details in the README.
(The nix toolchains are defined in non_module_deps.bzl using nixpkgs_go_configure).