feat(pgx-v5): Add static typing support for jsonb_build_object

[m-ali-akbay]

Adds a sqlc.jsonb_build_object."Name"(key, value, ...) directive for returning JSON-shaped results that decode straight into a Go struct. It's handy for pulling a one-to-many relationship into a single query instead of running one query per parent row.

-- name: GetAuthors :many
SELECT
  authors.id,
  ARRAY(
    SELECT sqlc.jsonb_build_object."Book"('id', books.id, 'title', books.title)
    FROM books WHERE books.author_id = authors.id
  ) AS books
FROM authors;

gives you:

type GetAuthorsRow struct {
	ID    int64
	Books []Book
}

type Book struct {
	ID    int64  `json:"id"`
	Title string `json:"title"`
}

A few notes:

• pgx/v5 only — generate fails with a clear error for any other sql_package. pgx decodes the jsonb value into the struct (or []struct inside ARRAY(...)) directly, so there's no Scan/Value plumbing.
• The struct name is required and read off the qualified call — Postgres parses sqlc.jsonb_build_object."Book" as catalog.schema.name, so no new parser work.
• Nesting works: a value can be another jsonb_build_object or an ARRAY(...) of one.
• Two queries that use the same name share one Go type when their shapes match; a shape mismatch or a collision with a model fails generation. Names and fields can be overridden with rename.
• Internally the call is just rewritten to Postgres's jsonb_build_object before analysis.

Docs in docs/howto/embedding.md; covered by unit tests plus a sqlc_json golden fixture.