Add 'type handlers' that allow users to easily support types we don't handle

[bloeys]

Hi Raphael,

I have made a small addition that allows users to inject 'handlers' at runtime to add support for types that aren't handled by default. As an example of how this is used, I have made jai-pgvector, which adds support for the vector type from the popular pgvector extension, which we need in our company.

This should also make it very easy for others to add support for other types and other extensions, like PostGIS, for example. If you like, we can also use this ability to add some of the more popular types and extensions in Jaipg itself, as an on-demand feature (e.g. people who want pgvector call init_pgvector or similar).

Wish you a nice day 😊

[rluba]

Thanks, this looks good! Do you have a sample implementation for any type that we could add to examples/example.jai to (a) document how it works and (b) use as a regression test?

[bloeys]

Regarding a reference implementation, currently the only one I have is jai-pgvector, which is 1 file of ~130 lines. Perhaps we can add that as a module and use it in the example, and reference it?

[bloeys]

I added usage of registering a handler in the examples that uses jai-pgvector.

Tell me if you would like any other changes :)