[mlir-lsp] Add DiagnosticTag from LSP spec (#91396)

Adds the [DiagnosticTag][diagtag] LSP construct to the LSP support
headers. I also added a unit test file to validate that the `tags` array
is omitted entirely if it's empty.

The LSP spec requires that `Diagnostic::tags` be an array; in order to
conform to that I used `std::vector`, as `SmallVector` doesn't have JSON
decoding support (you can encode it to JSON, but not decode it from
JSON).

[diagtag]:
https://microsoft.github.io/language-server-protocol/specifications/lsp/3.18/specification/#diagnosticTag