feat: rust embedding registry (#1259)

Todo:

- [x] add proper documentation
- [x] add unit tests
- [x] better handling of the registry**1
- [x] allow user defined registry**2

**1 The python implementation just uses a global registry so it makes
things a bit easier. I attached it to the db/connection to prevent
future conflicts if running multiple connections/databases. I mostly
modeled the registry & pattern off of datafusion's
[FunctionRegistry](https://docs.rs/datafusion/latest/datafusion/execution/trait.FunctionRegistry.html).

**2 Ideally, the user should be able to provide it's own registry
entirely, but currently it just uses an in memory registry by default
(_which isn't configurable_)

`rust/lancedb/examples/embedding_registry.rs` provides a thorough
example of expected usage.

---

Some additional notes:

This does not provide any of the out of box functionality that the
python registry does.

_i.e there are no built-in embedding functions._ 

You can think of this as the ground work for adding those built in
functions, So while this is part of
https://github.com/lancedb/lancedb/issues/994, it does not yet offer
feature parity.

.gitignore

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