x/tools/gopls: temporarily reinstate the 'Structured' hoverkind.

[findleyr]

Unfortunately, our audit of usage of the misguided experimental "structured" hoverkind setting failed to identify that vim-go used it, and we bypassed our normal probationary period for removing settings.

We should add this setting back until dependent clients can be updated, with a warning and telemetry.

FWIW, vim-go was using Structured.

Originally posted by @bhcleek in #70233

[findleyr]

Argh, I failed to add json struct tags in the partial revert. Reopening.

[findleyr]

(reopened wrong issue)