Fix: Image Block: Add protocol to custom link

[donmhico]

Description

Append HTTP protocol to custom link set in Image Block.

How has this been tested?

1. Create a post.
2. Insert an Image Block and select image from library.
3. In the Toolbar click "Insert Link" icon.
4. Enter an address. (Tested addresses with http, https, as well as without e.g. google.com.)
5. Click the "external link" icon besides the custom link. (It will redirect you to the custom URL instead of a relative URL).
6. Save the post and view it in the frontend.
7. Click the image. (It will now redirect you to the custom URL instead of a relative URL).

Worth consideration

The change uses prependHTTP which will always prepend HTTP. It might be worth considering prepending HTTPS instead.

Types of changes

Fixes #17155.

Checklist:

• My code is tested.
• My code follows the WordPress code style.

[senadir]

this PR looks good expect I can't get it to work because this doesn't seem to be set to true linkDestination === LINK_DESTINATION_CUSTOM.

Also, wouldn't make sense to add //: instead of http:// to allow both normal and secure protocols to exist depending on the website. I would go as far as only allowing https protocols, but this is for another discussion.

[draganescu]

This doesn't seem to work @donmhico Would you have some time to revise and test this PR?

[annezazu]

Closing this PR out due to lack of responses. Feel free to re-open and continue working on this though 🥳