This adds a new HTTP-specific processor that formats logs according to the HttpRequest Google Cloud Logging log format.

See: https://cloud.google.com/logging/docs/structured-logging#structured_logging_special_fields
See: https://cloud.google.com/logging/docs/reference/v2/rest/v2/LogEntry#httprequest

Fix: #32

This introduces:

• A new class to represent the current HTTP Request. It maps to what GCP supports, but with specific attributes which are a bit more agnostic (size is a number, not a string, for instance.)
• A new processor that "knows" how to convert the previous HTTP request object for logging

This can be used in different ways:

Either the user creates the HTTPRequest object itself and logs it directly:

def handler(request):
    r = HTTPRequest(method=request.method, ...)
    logger.info("new request coming", http_request=r)

Or this is implemented as part of a framework (inside WSGI or ASGI or framework-specific middleware for instance), in which case we can offer a dedicated adapter from the framework objects to the HTTPRequest format.

For this case, the pull request offers an adapter from Starlette request/response + a test with a basic Starlette middleware.