Feature Request: worker entrypoint support for custom file handles

[DaveShuckerow]

Hi! I'm a prospective user exploring duckdb-wasm as a query engine for OPFS files.

I'd like to have the ability to inject custom handles into duckdb's file registration.

This could be done by allowing users to write their own duckdb entrypoint that instantiates whichever worker (mvp, eh, etc), injecting implementations of filehandle factories for different file protocols. Duckdb could then use the those when trying to get a handle for a file.

My primary interest in this feature is that it would let duckdb not have to worry about staying up-to-date on the latest OPFS changes (I'd be able to deal with #2066 in my application logic, for instance).

But it would also reduce the maintenance overhead duckdb-wasm would have to be concerned with for other specialized types of file access.

For example, this would also be helpful for things like the S3 signed URL bug, or any other situation where a user has access to a file through some custom means. For example, maybe they have their own API server for the file contents, or they're using a file protocol not yet supported by duckdb-wasm.