feat: support vector subscript

[usamoi]

Address #327

Now we support such a syntax:

CREATE TABLE t(val vector(3));
INSERT INTO t (val) SELECT ARRAY[random(), random(), random()]::real[] FROM generate_series(1, 100000);
CREATE INDEX t_vector_index on t using vectors((val[:1]::vector(1)) vector_l2_ops);
SELECT * FROM t ORDER BY val[:1]::vector(1) <-> '[0]' limit 10;

It's only supported by pg14, pg15 and pg16.

[VoVAllen]

The syntax using real[] is not consistent with other usage with vectors. Is it necessary?
Is vectors a new function?

[usamoi]

The syntax using real[] is not consistent with other usage with vectors.

It's nothing about real[]. We store vectors in a column. This PR supports subscripts of value of vector type.

Is vectors a new function?

It's vectors AM handler. We create indexes on column val before and we create indexes on expression val[:1]::vector(1) now. You can get reference of syntax in https://www.postgresql.org/docs/current/sql-createindex.html.

[VoVAllen]

Can I do CREATE INDEX t_vector_index on t using vectors((val[16:32]::vector(16)) vector_l2_ops); ?

[usamoi]

Can I do CREATE INDEX t_vector_index on t using vectors((val[16:32]::vector(16)) vector_l2_ops); ?

Yes.