Support base64-encoded data

[rnro]

This change accompanies apple/swift-openapi-runtime#55 and relies on it for the OpenAPIRuntime.Base64EncodedData type.

Motivation

OpenAPI supports base64-encoded data but to this point Swift OpenAPI Generator has not (#11).

Modifications

A data type specified as type: string, format: byte will now result in a generated type which is Codable and backed by a OpenAPIRuntime.Base64EncodedData type which knows how to encode and decode base64 data.

Result

Users will be able to specify request/response payloads as base64-encoded data which will be encoded and decoded transparently

Test Plan

Unit tested locally.

[czechboy0]

@swift-server-bot add to allowlist

[czechboy0]

@rnro As a last thing, can you add a base64 property to an object of your choice in the file-based reference tests? I'm a little paranoid that this PR is passing even though we haven't released the update runtime yet, which means we're not testing that it all builds and works together. Add a property, and make sure PetstoreConsumerTests are updated to also populate and verify the value both on client and server side. Otherwise lgtm.

[czechboy0]

One last ask about testing, see my other comment.

[rnro]

Thanks for the review, I've added an endpoint to upload audio of your pet in base64-encoded binary format. I'm not sure if that's the best API design for an actual pet app but I think it's plausible to get the new code in there.

[czechboy0]

Please also add tests to Test_Client and Test_Server that set and verify the value in Pet.genome, just to make sure it works end-to-end.

[czechboy0]

Also, you can now update to 0.3.2 of the runtime library in Package.swift of swift-openapi-generator, all that should hopefully make the CI green on this PR.

[czechboy0]

Looks great, thank you @rnro!