ESQL: introduce mapping-unavailable fields grammar

[bpintea]

This adds the grammar for the mapping-unavailable fields: the FROM command gets two new clauses, similar to METADATA: OPTIONAL and UNMAPPED.

These new clauses can receive field names, field name patterns or identifier parameter markers.

They can be provided in any order (along with METADATA), but each can only appear at most once.

Examples:

FROM test OPTIONAL specificFoo, `metadata`, optionalFields* METADATA _index UNMAPPED specificBar, `unmapped`
FROM test UNMAPPED *
FROM test OPTIONAL ??param1, embeddedFieldName

Related: #138888

[elasticsearchmachine]

Hi @bpintea, I've created a changelog YAML for you.

[bpintea]

Started with #139417, will re-evaluate the need this PR adrdesses at a later time.