`.entities()` typedef update

## Overview
Currently, the return type definitions for `.entities()` methods are inaccurate. it says `Iterable[CURIE]`, but it is also possible for it to return "quoted URIs" (example: ``).

## Possible solutions
a. Fix so that URIs, not quoted URIs are returned, and update type def `Union[CURIE, URI]`
b. Add new type for quoted URIs and update type def `Union[CURIE, QUOTED_URI[`
c. `IDENTIFIER`
- rename `CURIE` to `IDENTIFIER`, such that this is used universally in signatures. This will be a large diff, but it will not affect runtime
- Formally define this as `IDENTIFIER = CURIE | QUOTED_URI`. Again, this is typing information that does not affect runtime, but it helps make the intentions clear

## Additional details
Context / discussion

_[Chris](https://github.com/INCATools/ontology-access-kit/pull/688#issuecomment-1863151973)_:
> The intent is that CURIEs are input and output. Yes, I know that when using semsql URIs that have no prefixes get turned into quoted URIs (different from URIs), but this should be fixed elsewhere.

_[Joe](https://github.com/INCATools/ontology-access-kit/pull/688#issuecomment-1863524743):_
> I guess you're right, they are "quoted URIs". Here's an example of one I see:
> > `''`
>
> Why don't we add something like QUOTED_URI to `oaklib.types`?:
> ```py
> CURIE = str
> QUOTED_URI = str
> URI = str
> ```
>
> When you say "should be fixed elsewhere", I assume you are referring to them being "quoted" URI strings rather than plain URI strings. I'm definitely down with that. If you mean that URIs should never be returned (you probably don't), I don't think that's possible 100% of the time.

## Related
- #684

Start with the `.entities()` method signatures and the type definitions in `oaklib.types`, then review the discussion in pull request #688 and related issue #684. The work is complete when the intended return type for `.entities()` is decided and the affected type definitions and annotations consistently reflect it.

Written by the indexing model from the issue text.