fix(libsql-sys): support non-Unix when rusqlite feature is disabled

[CBenoit]

Currently the codepath used when rusqlite is disabled, is Unix-platform-dependent.

For instance, here is the error we compiling from Windows:

error[E0433]: failed to resolve: could not find `unix` in `os`
   --> C:\Users\runneradmin\.cargo\git\checkouts\libsql-311658d335deb3b1\9b04f1d\libsql-sys\src\connection.rs:276:26
    |
276 |             use std::os::unix::ffi::OsStrExt;
    |                          ^^^^ could not find `unix` in `os`
    |
note: found an item that was configured out
   --> /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14\library\std\src\os\mod.rs:27:9
note: the item is gated here
   --> /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14\library\std\src\os\mod.rs:19:1
note: found an item that was configured out
   --> /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14\library\std\src\os\mod.rs:65:9
note: the item is gated here
   --> /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14\library\std\src\os\mod.rs:64:1

error[E0599]: no method named `as_bytes` found for reference `&OsStr` in the current scope
   --> C:\Users\runneradmin\.cargo\git\checkouts\libsql-311658d335deb3b1\9b04f1d\libsql-sys\src\connection.rs:277:73
    |
277 |             let path = std::ffi::CString::new(path.as_ref().as_os_str().as_bytes())
    |                                                                         ^^^^^^^^
    |
help: there is a method `as_encoded_bytes` with a similar name
    |
277 |             let path = std::ffi::CString::new(path.as_ref().as_os_str().as_encoded_bytes())
    |                                                                         ~~~~~~~~~~~~~~~~

Some errors have detailed explanations: E0433, E0599.
For more information about an error, try `rustc --explain E0433`.
error: could not compile `libsql-sys` (lib) due to 2 previous errors

This patch is adding a platform-independent branch.

The database path is expected to be UTF-8 by the libsql native library. However, in the real world, all Unix paths are not necessarily UTF-8. For this reason, I understand why the Unix codepath is attempting a direct conversion from OsString to CString.
However, it’s not possible to do something similar on other platforms. For these platforms, we are enforcing correct UTF-8 using to_str. At least, it should work reasonably well in most cases.

[CBenoit]

Hello @penberg
I’m not sure how my change is impacting the "Go bindings tests" job.

[penberg]

@CBenoit Please rebase on top of main, you're just getting hit by unrelated bug that is fixed in main.

[CBenoit]

@penberg I see! Done!