Create Image::to_base64

[boswelja]

This opens up the possibility of models sending back images, and clients being able to get the image contents as base64.
I've also updated from_base64 to take an impl Into<String>, meaning the function will be happy to take an &str or a String, and run cargo clippy --fix

[boswelja]

I wasn't sure what the preferred behavior is here 0 as the PR is now it'll give you a borrowed reference to the images base64 representation, but I can see value in having to_base64() consume the Image and give a String, ex

    pub fn to_base64(self) -> String {
        self.0
    }
    ```