support form data in services

[cataggar]

A bit like https://docs.rs/reqwest/0.11.6/reqwest/multipart/struct.Form.html
But needs to support wasm too

[ctaggart]

https://swagger.io/docs/specification/2-0/describing-parameters/#form-parameters

If you search the specifications for formData, it returns 108 results in 29 files.

Code generation currently writes unimplemented. An example:

    Checking azure_svc_containerregistry v0.1.0 (/Users/runner/work/azure-sdk-for-rust/azure-sdk-for-rust/services/svc/containerregistry)
error: unreachable statement
    --> svc/containerregistry/src/package_2019_07/operations.rs:1617:17
     |
1616 |                 unimplemented!("form data not yet supported");
     |                 --------------------------------------------- any code following this expression is unreachable
1617 |                 unimplemented!("form data not yet supported");
     |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unreachable statement
     |
     = note: `-D unreachable-code` implied by `-D warnings`
     = note: this error originates in the macro `$crate::panic` (in Nightly builds, run with -Z macro-backtrace for more info)

[juliusl]

@cataggar Is this an item I could implement?