feat(cube): support postgres cube

[jayy-lmao]

I work on a sqlx codegen tool, and had an issue posted about the lack of support for the 'cube' extension type (https://www.postgresql.org/docs/current/cube.html).

jayy-lmao/sql-gen#9

I could not see any outstanding issues / PRs involving Cube, so can create an issue first if necessary for discussion.
Not particularly attached to how to structure the struct, serialisation, or deserialisation - ultimately just want to be able to support it for my sql-gen tool so that @yellowHatpro can carry on with using it for their project.

[jayy-lmao]

Sorry for the build jobs -- running locally the encoding seems to be fine.
Haven't been able to replicate locally with amd64 platform, alpine etc. Still figuring out why the encoding is different.

Update: have been able to reproduce by using

    let row = s.try_next().await?.unwrap();
    let rec = row.try_get::<PgCube, _>(0)?;

Locally to test. Will continue to investigate.

[jayy-lmao]

Have handled the text case, tests now passing ☺️

[jayy-lmao]

Have made the changes to vector indexing :)

[abonander]

This should be a FromStr impl.

It's fine to have both, but it's surprising to have a TryFrom<&str> impl and not a FromStr impl.

The only reason to do that is if Self wants to borrow from the string (think zero-copy parsing) which the FromStr trait doesn't support due to its lack of a lifetime bound.

[jayy-lmao]

Have made changes

• trimming the input string to make it more flexible for the spec & for user input
• allow for _cube array type (And included a test for it)
• remove TryFrom behaviour for &[u8], using direct function instead
• avoid double creating a buffer, but just writing directly to the PgArgumentBuffer

[jayy-lmao]

Apologies, I've been away for a little - I've addressed all comments but if there's any other changes that are wanted let me know