Improved documentation for usage in `wasm32-unknown-unknown`

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**

The docs [currently say](https://github.com/apache/arrow-rs-object-store/blob/12ef9bc22c56228b25f3b56a78879e9a45a07fc4/README.md?plain=1#L45-L51)

> ## Support for `wasm32-unknown-unknown` target
>
> It's possible to build `object_store` for the `wasm32-unknown-unknown` target, however the cloud storage features `aws`, `azure`, `gcp`, and `http` are not supported.
>
> ```
> cargo build -p object_store --target wasm32-unknown-unknown
> ```

Am I correct that this assertion is outdated? If the user implements a custom `HTTPConnector` in wasm32, then one can use aws/azure/gcp/http in wasm32, right?

If so, I think we should improve the documentation here, even if we don't have a full example working in wasm32.

**Describe the solution you'd like**

Improved documentation

**Describe alternatives you've considered**

**Additional context**

Start with the wasm32-unknown-unknown section in README.md and verify the documented limitations against the crate's HTTPConnector behavior for custom connectors. Use the README's cargo build command as a baseline; done means the documentation accurately explains whether aws, azure, gcp, and http can be used with a custom connector in wasm32, without requiring a full example.

Written by the indexing model from the issue text.