Proposal: Add variant of `CustomizeDiffFunc` that returns `diag.Diagnostics`

### Use-cases

There are times when a `CustomizeDiffFunc` would ideally return a warning message. The current option is to add a log message, but that is only visible to the practitioner if logging is enabled.

### Proposal

Create a variant of `CustomizeDiffFunc`, e.g., `CustomizeDiffFuncDiags`, that returns a `diag.Diagnostics` instead of `error`.

Start by locating CustomizeDiffFunc and the existing diag.Diagnostics type in the SDK. Trace how CustomizeDiffFunc results are handled, then define done as supporting a diagnostics-returning variant that can expose warning messages to practitioners.

Written by the indexing model from the issue text.