[rust-server] fix bug that requires binary string to be encoded as base64.

Created by: x7c1

fix #3189 (and some minor bugs described below.)

Steps to reproduce

# generate petstore examples
$ java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate \
   -i https://raw.githubusercontent.com/openapitools/openapi-generator/master/modules/openapi-generator/src/test/resources/3_0/petstore.yaml \
   -g rust-server \
   -o /var/tmp/rust-server

# launch rust-server
$ cd /var/tmp/rust-server
$ RUST_LOG=debug cargo run --example server
    Finished dev [unoptimized + debuginfo] target(s) in 0.06s
     Running `target/debug/examples/server`

# upload any file
$ curl -v -F 'file=@README.md' localhost:8080/v2/pet/1/uploadImage                     
*   Trying 127.0.0.1:8080...
...
file data does not match API definition : expected value at line 1 column 1%

This is because format: binary is decoded as base64 (#3189).

I've confirmed this PR can remove this error :

$ curl -F 'file=@README.md' localhost:8080/v2/pet/1/uploadImage                     

# server outputs (fixed)
[2021-09-12T04:04:35Z INFO  server::server] upload_file(1, None, Some(ByteArray([...]]))) - X-Span-ID: "14d8e6f4-1c84-4319-9f42-5e15ee70e673"

This PR can remove another error which rejects the form fields like additionalMetadata=asdf.

$ curl -F 'file=@README.md' -F 'additionalMetadata=asdf' localhost:8080/v2/pet/1/uploadImage           

# server outputs (fixed)
[2021-09-12T10:31:47Z INFO  server::server] upload_file(1, Some("asdf"), Some(ByteArray([...]]))) - X-Span-ID: "1b3e1089-fe74-4e61-b69b-3984654a4d0f"

This error had been caused by following two server bugs which

• A) had accepted paramName (additional_metadata), not baseName (additionalMetadata).
• B) had decoded primitive values (on multipart form data) as JSON. (for example, it had only accepted foo="asdf" format string, not foo=asdf.)

Rust Technical Committee

@frol @farcaller @richardwhiuk @paladinzh

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