Add support for application/xml body

[ugocottin]

Motivation

See apple/swift-openapi-generator#556 for more.

Modifications

Add converter methods for encoding and decoding XML request and response body.
Add CustomCoder protocol, allows to use other Encoder and Decoder for other content-type body.

User can define custom coder, and assign a custom coder to a specific content-type within Configuration.customCoders dictionary.

Result

It's now possible to define custom encoder and decoder for supported content-type.

Test Plan

Added converter methods are tested with a mock custom coder for application/xml content-type. To avoid adding a dependency to a XMLCoder like CoreOffice/XMLCoder, mock custom coder uses JSONEncoder and JSONDecoder.

Encoding and decoding to XML are out of scope of the tests, because encoding and decoding logic must be provided by user through custom coder implementation.

[czechboy0]

Looks really good! A few initial comments, mostly minor suggestions.

[czechboy0]

@swift-server-bot test this please

[ugocottin]

@czechboy0 I've made the suggested changes, the PR on swift-openapi-generator will follow soon, thanks

[czechboy0]

@swift-server-bot test this please

[czechboy0]

@swift-server-bot test this please

[ugocottin]

Jenkins CI reports that OpenAPIMIMEType struct is not Sendable, and must be to be use in RuntimeError.missingCoderForCustomContentType(contentType:).

OpenAPIMIMEType seems thread safe to me. Should I make OpenAPIMIMEType (and OpenAPIMIMEType.Kind) conform to Sendable, or use String type in RuntimeError.missingCoderForCustomContentType(contentType:) ?

[czechboy0]

Just convert it to string when constructing the error.

[ugocottin]

@swift-server-bot test this please

[czechboy0]

@swift-server-bot test this please

[czechboy0]

@swift-server-bot test this please

[czechboy0]

Looks great, @ugocottin - will merge after CI passes. Feel free to move onto the generator PR. We'll do a release of the runtime early next week, at which point your generator PR should start passing on CI.