importer: handle IMPORT options at parse time with uniqueness checks

**Summary:**
IMPORT options are evaluated into a `map[string]string` via
`exprutil.Evaluator.KVOptions`, so a duplicated option is silently last-value-wins
with no parse-time uniqueness check — unlike BACKUP/RESTORE, which handle options
at parse time and error on duplicates. Migrate IMPORT to the same parse-time model.

**Background:**
The `execution_locality` option is a generic KVOption (chosen for uniformity with
the other IMPORT options). The tradeoff is that it — and every other IMPORT option
— is last-value-wins on duplicates. This tracks moving all IMPORT options to
parse-time handling for uniform uniqueness detection.

**Proposed work:**
- [ ] Handle IMPORT options at parse time (grammar or a parse-time combiner),
matching BACKUP/RESTORE.
- [ ] Restore duplicate-detection for `execution_locality` and add it for the
other options.

Epic CRDB-65504

Jira issue: CRDB-66933

Compare IMPORT option handling with the parse-time path used by BACKUP/RESTORE, then inspect exprutil.Evaluator.KVOptions. Done means IMPORT options are handled at parse time and duplicate execution_locality and other options are rejected rather than silently using the last value.

Written by the indexing model from the issue text.