Add support of Vision API.

[kstefanou52]

Adds support for the Vision API, specifically the Annotate Image endpoint.

Usage:

 func annotateImage(_ imageURL: URL) throws -> EventLoopFuture<VisionAnnotateImageResponse> {
        let credentials: GoogleCloudCredentialsConfiguration(...)
        let config: GoogleCloudVisionConfiguration(...)
        let httpClient = HTTPClient(...)
        
        let visionClient = try GoogleCloudVisionClient(
            credentials: credentials,
            config: config,
            httpClient: httpClient,
            eventLoop: eventLoop
        )
        
        let imageRequest = AnnotateImageRequest(imageUri: imageURL.absoluteString)
        
        return visionClient
            .vision
            .annotateImage(imageRequest)
    }