Add option to pass HTTP headers to API calls

[kovan]

Suggested implementation for #79.
Tested and working.

Summary:

Each Google API response has a E-tag attached to it, which is included in the JSON response.
If you save the response along with this E-tag, you can later check if the content of the response that you previously saved has has changed or not on the server, by adding the "If-None-Match" HTTP header to the API call. If the content has not changed, the API server answers with the code "304 not modifed".

This is a simple way to keep data retrieved from the API up to date.

The code of the API call would look like this:

    async with Aiogoogle(
        api_key= "your-api-key"
    ) as aiogoogle:
        api = await aiogoogle.discover("youtube", "v3")
        # etag hardcoded for the example, but this is supposed to be retrieved from the database or similar:
        etag = "vsu1KWWbOO-15Idit0GDZKgEFfo" 
        result = await aiogoogle.as_api_key( 
            api.videos.list(
                headers={"If-None-Match": etag},
                part="snippet,contentDetails,statistics",
                id="YOUR_VIDEO_ID"
            ),
            full_res=True
        )
        if result.status_code == 304:
            print("Resource was not modified since last check")

[omarryhan]

Hey, so sorry for the late response.
I'm hesitant to merge this in because if we pass the keyword headers here, it might conflict with a potential API argument with the same name. Can we maybe pass it inside a kwarg called aiogoogle_request_options? e.g.

api.videos.list(
    aiogoogle_request_options={
        "headers": {"If-None-Match": etag}
    },
    part="snippet,contentDetails,statistics",
    id="YOUR_VIDEO_ID",
)

[kovan]

Whatever works, OK. Works well for me.