x/tools/gopls: story for custom codelens providers

[hyangah]

gopls started to offer test/benchmark codelenses
for test and benchmark functions that work with the testing package.

The VS Code Go extension that has offered this feature for a while in its own way received multiple
feature requests to support various testing frameworks (e.g. gocheck, ginkgo, testify, ...)
and we expect to get more as more custom testing frameworks show up.

Current VS Code Go's plan is to export necessary symbol information to other extensions, so
these feature requests can be implemented as separate extensions.

However, I think it is more desirable for long term if this type of feature extension works more closely
with gopls. That will allow other editors to benefit from the work, and also more go developers
can contribute.

This is also related to the question of how to support various linters, custom diagnostics providers.
(I couldn't find an open issue about custom linter support yet)