DAST Site validation - Improve validation path handling

Problem to solve

When creating a DAST Site profile, users are asked to provide a Target URL which can then be validated by switching the Validate target site toggle on. In the validation section that shows up, one of the options available involves uploading a validation file to the target site, and providing the file's URL via a text input with the Target URL prepended.

The current iteration doesn't actually uses the validation file's location at all. Let's make sure users are able to provide a path to their validation file based on this discussion.

• The prefix should not be the full Target URL, it should only consist of the protocol and the resource name (e.g. if the Target URL is https://gitlab.com/some/path/, the prefix should be https://gitlab.com/)
• The validation file path input should default to the generated validation file's name.
• If the Target URL contains a file path, it should be prepended to the validation file's name in the input (e.g. if the Target URL is https://gitlab.com/some/path/, and the generated file name is validation_token.txt, the input defaults to some/path/validation_token.txt.
• Finally, the prefix should always end with a slash (e.g. if the Target URL is https://gitlab.com, the prefix should be https://gitlab.com/.