Install pg_vector extension with pgxman

[owenthereal]

What's changed?

• Install pg_vector extension with pgxman. Note that it's intentional to always force installing extensions with pgxman for now to avoid Docker layer caching in case there are any updates made to the extension packages.
• Add acceptance tests to validate pg_vector is working as expected.

A demo of pg_vector running with the built image:

postgres=# create extension vector;
CREATE EXTENSION
postgres=# CREATE TABLE items (id bigserial PRIMARY KEY, embedding vector(3));
CREATE TABLE
postgres=# INSERT INTO items (embedding) VALUES ('[1,2,3]'), ('[4,5,6]');
INSERT 0 2
postgres=# SELECT * FROM items ORDER BY embedding <-> '[3,1,2]' LIMIT 5;
 id | embedding 
----+-----------
  1 | [1,2,3]
  2 | [4,5,6]
(2 rows)

[JerrySievert]

wow, that's some backward compatibility!

[JerrySievert]

lgtm

[wuputah]

fine for now, but we need to think about how to get out of this business of adding stuff to this setting