Run codegen for first party Objective-C to Swift RPC migration shim

[julianlocke]

Existing integration tests not expected to pass. This code to be deleted post migration.

Stone changes: dropbox/stone#350

This PR creates a migration shim for first party use. The intention of the shim is to facilitate a feature-gated migration from ObjectiveDropbox to SwiftyDrobpox. Specifically it is to allow broad usage of SwiftyDropbox without modifying ObjectiveDropbox call sites.

There are four pieces that make up the shim:

1. API object mappers from Objective-C (ObjectiveDropbox) types to Swifty Objective-C compatibility layer types

func mapDBFILESCommitInfoToDBX(object: DBFILESCommitInfo) -> DBXFilesCommitInfo {
    return DBXFilesCommitInfo(path: object.path,
        mode: mapDBFILESWriteModeToDBX(object: object.mode),
        autorename: object.autorename,
        clientModified: object.clientModified,
        mute: object.mute,
        propertyGroups: object.propertyGroups?.compactMap { mapDBFILEPROPERTIESPropertyGroupToDBX(object: $0) },
        strictConflict: object.strictConflict,
        fswRequest: mapDBFILESFswRequestToDBXOptional(object: object.fswRequest),
        fileEncryptionInfo: mapDBENCRYPTIONEncryptedFileToDBXOptional(object: object.fileEncryptionInfo)
    )
}

2. API object mappers from Swifty Objective-C compatibility layer types to Objective-C (ObjectiveDropbox) types

func mapDBXFilesCommitInfoToDB(object: DBXFilesCommitInfo) -> DBFILESCommitInfo {
    return DBFILESCommitInfo(path: object.path,
        mode: mapDBXFilesWriteModeToDB(object: object.mode),
        autorename: object.autorename,
        clientModified: object.clientModified,
        mute: object.mute,
        propertyGroups: object.propertyGroups?.compactMap { mapDBXFilePropertiesPropertyGroupToDB(object: $0) },
        strictConflict: object.strictConflict,
        fswRequest: mapDBXFilesFswRequestToDBOptional(object: object.fswRequest),
        fileEncryptionInfo: mapDBXEncryptionEncryptedFileToDBOptional(object: object.fileEncryptionInfo)
    )
}

3. A function that provides a Swifty api request when provided with Objective-C api request context

@objc public static func rpcRequest(for route: DBRoute, args: DBSerializable, userId: String, client: DBXDropboxClient) -> DBXRequest? {
    // Make sure this handles versioned routes properly
    let qualifiedRouteName = route.namespace_ + "/" + route.name
    if qualifiedRouteName == "account/change_name", let args = args as? DBACCOUNTAccountNameArg {
        return client.account.changeName(firstName: args.firstName, lastName: args.lastName)
    }
    if qualifiedRouteName == "account/delete_account", let args = args as? DBACCOUNTDeleteAccountArg {
    etc.

4. A function that allows you to set an Objective-C response block on a Swifty api request and maps the Swifty API objects received back to Objective-C objects.

@objc public static  func setResponseBlockRPC(block: @escaping DBRpcResponseBlockImpl, on task: DBXRequest, with queue: OperationQueue) -> Bool {
    if let task = task as? DBXAccountChangeNameRpcRequest {
        task.response { routeError, networkError in
            let mappedError = mapDBXAccountChangeNameErrorToDBOptional(object: routeError)
            let mappedCallError = callErrorToDB(error: networkError)
            let wrappedBlock = { block((networkError == nil && routeError == nil) ? DBNilObject() : nil, mappedError, mappedCallError) }
            Self.processCompletion(wrappedBlock, queue: queue)
        }
        return true
    }
    if let task = task as? DBXAccountDeleteAccountRpcRequest {
    etc.

Usage:
Our ObjectiveDropbox transport client subclass will direct all requests for an Objective-C RPC API request to the shim instead, if gated. Similarly a new RPCTask subclass will route response blocks to the shim and cancellations to the underlying swift api request object.