[clangd] Introduce config compilation for External blocks

Compilation logic for External blocks. A few of the high level points:
- Requires exactly one-of File/Server at a time:
  - Server is ignored in case of both, with a warning.
  - Having none is an error, would render ExternalBlock void.
- Ensures mountpoint is an absolute path:
  - Interprets it as relative to FragmentDirectory.
  - Defaults to FragmentDirectory when empty.
- Marks Background as Skip.

Depends on D90748.

Differential Revision: https://reviews.llvm.org/D90749