x/tools/gopls: consider static configuration for multiple builds

[findleyr]

This is a follow up to #29202. In that issue, we added dynamic support for tracking multiple GOOS/GOARCH combinations, based on the set of open files. (e.g. if you open foo_windows.go, gopls will create a new View with GOOS=windows).

In some cases, it may be desirable for users to explicitly configure these builds, for example if they know a priori that they are targeting several ports, or sets of build tags. We should consider supporting this, for example via configuration like so:

"builds": [{
  "env": ["GOOS=windows", "GOWORK=off"],
  "flags": ["-tags", "example"]
}, {
  // another build...
}]

[bersace]

I want to use custom build tags to distinguish unit tests from e2e test as described in https://blog.alexellis.io/golang-e2e-testing-case-study/. However, gopls is lost with no package metadata for file. How to make gopls aware of tags ?