Support base64 encoded data (type: string + format: byte)

[czechboy0]

Currently, only type: string + format: binary is supported, and represents raw data (an octet stream).

OpenAPI also supports base64-encoded data, represented as the following JSON schema:

type: string
format: byte

The generated type for it should also be Foundation.Data (same as for format: binary), but as part of serialization/deserialization, the extra step of base64 coding needs to be added.

Testing: verify that both a whole request/response body being application/json with the contents of a base64-encoded string (use case 1), but also as a nested field in a JSON object (use case 2), both are pretty common.

[czechboy0]

Note that since this isn't a content type, but a way to encode and decode any schema, including a nested one, this'll likely justify creating a type looking like this in the runtime library:

struct Base64EncodedData: Sendable, Codable, Hashable {
    var data: Foundation.Data // or [UInt8] or ArraySlice<UInt8>
    init(data: Foundation.Data) {
        self.data = data
    }
    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        let base64EncodedString = try container.decode(String.self)
        guard let data = Data(base64Encoded: base64EncodedString) else {
            throw RuntimeError.invalidBase64String(base64EncodedString)
        }
        self.init(data: data)
    }
    func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        let base64String = data.base64EncodedString()
        try container.encode(base64String)
    }
}

And then for e.g. the following JSON Schema snippet

MyObj:
  type: object
  properties:
    stuff:
      type: string
      format: byte
  required: [stuff]

generate a struct like:

struct MyObj: Sendable, Codable, Hashable {
  var stuff: OpenAPIRuntime.Base64EncodedData
  // ...
}