Support for signed URLs

[profitrollgame]

After using the plugin unsigned for some time, the lack of URL signatures felt noticeable to me.

In short, the following changes were made:

• Added options imgproxy_key and imgproxy_salt
• Added fields for both options in the settings
• Added both options to Imgproxy and ImgproxyImage
• Modified ImgproxyImage.url() to use ImgproxyImage.get_path_signature() for URL signing
• Disabled signing with a fallback to i instead of a signature if either key or salt is null or an empty string

This feature was tested on my WordPress 6.8.1 website, both with and without a signature.

I'm new to WordPress plugins and PHP in general, so I'm sure that some things could have been done better.
Unfortunately, I do not have enough knowledge to improve this feature much further now :(

[uhlhosting]

@joshbetz this project dead?