LSP API Diagnostic should support an unique ID

### Description

An `ErrorProvider` must create Diagnostic objects through its Builder, so it cannot control (maintain) the identity easily. But it needs to somehow allow to pair previous and current objects, so that LSP core knows what diagnostics are just published again, and which Diagnostic are not "duplicates".

For Java errors and hints, a line-column info is embeded into `Diagnostic.`code property, which is not good, as the `code` is displayed in the UI - and for example line:column information is duplicated.

The SPI implementor should be able to provide an ID which will be used for matching, separately from the diagnostic code.

### Use case/motivation

_No response_

### Related issues

_No response_

### Are you willing to submit a pull request?

Yes

Start by tracing the ErrorProvider Diagnostic Builder and the LSP core matching behavior described in the issue. Review how Java errors and hints currently place line-column data in Diagnostic.code. Done means an SPI implementor can provide a separate diagnostic ID so previously published diagnostics can be matched without changing the displayed code.

Written by the indexing model from the issue text.