feat(services/fs): add file as an alias scheme for fs

[userzhy]

Which issue does this PR close?

Closes #7066.

Rationale for this change

The recently-added URI feature uses OpenDAL's existing idea of a scheme, which in most cases matches the commonly-used schemes for different services. However, the filesystem service uses the scheme fs, while standard file system URIs use the scheme file. This is confusing for users who expect to use file:// URIs.

By adding file as an alias for fs, users can use the standard URI scheme while maintaining backward compatibility with the existing fs scheme.

Note: I noticed @clbarnes indicated willingness to contribute on this issue. If you're already working on this, please let me know and I'm happy to close this PR.

What changes are included in this PR?

• Added FILE_SCHEME constant ("file") in core/services/fs/src/lib.rs
• Registered file as an alias for Fs service in the operator registry
• Added a test case to verify file:// URIs work correctly

Are there any user-facing changes?

Yes. Users can now use the standard file:// URI scheme to create filesystem operators, in addition to the existing fs:// scheme. This is fully backward compatible.

// Both of these now work:
let op1 = Operator::from_uri("fs:///tmp/data")?;
let op2 = Operator::from_uri("file:///tmp/data")?;

AI Usage Statement

This PR was developed with the assistance of Claude (Anthropic).

[clbarnes]

I hadn't started work on it so you're good to go! Thank you ♥️

[Xuanwo]

Thank you for working on this!