[rust] Added support for text/plain to reqwest clients

[ranger-ross]

I am continuing my quest to improve the Rust Reqwest based clients. :)

This PR adds better content type header handling to the reqwest & reqwest-trait clients.
Specifically it adds support for text/plain when the return type is String. (and better error messages when the return type is not a string, instead of blindly trying and failing to JSON parse non-json responses)

This will fix #20617 , however it will not fix hyper clients and if supportMultipleResponses is true then this approach will also not work.

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cc: @frol @farcaller @richardwhiuk @paladinzh @jacob-pro

[ranger-ross]

I would not call this a vendor extension since its not meant for the API spec to set x-supports-plain-text.
I just needed a way to set attach some arbitrary metadata to an operation and this was the easiest way that I found to do it.

If there is a better way to do this, please let me know.

[wing328]

in java client codegen, we've something similar:

openapi-generator/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/JavaClientCodegen.java

Line 825 in 9ed15a1

if ("text/plain".equalsIgnoreCase(produce.get("mediaType").split(";")[0].trim())

[wing328]

if i remember correctly, there was a use case before which the content type is application/json and the return type is string (e.g. "something here") so I think we will need to check the content type (produce) as well

[ranger-ross]

The rational for this logic is to be able to support both custom JSON variants and charset extensions.

• application/json
• application/vnd.github+json
• application/json; charset=utf-8
• application/vnd.github+json; charset=utf-8

as well as

• text/plain
• text/plain; charset=utf-8