Add splat and placeholder support to _redirects

What does this MR do?

Adds support for Netlify-style splats (*) and :placeholders in _redirects.

This can be used to route all matches to a single, static path:

/foo/* /bar 200

or to a dynamic path using the placeholders:

/foo/*                 /bar/:splat                     200
/foo/:year/:month/:day /bar/blog/:year-:month-day.html 200

Why?

There are a lot of use cases for this feature, but the killer feature for this is to support single page applications (SPAs) that expect the main index.html page to be served for all requests. (See #23 (closed))

This can be accomplished with a configuration like this:

/* / 200

Documentation

This new functionality will be documented in gitlab!59803 (merged)

TODO

• Added the Changelog (e.g. Changelog: feature) trailer to the commits that need to be included in the changelog
• Added unit tests (if required)
• Added acceptance tests (if required)
• Documentation (if required) (will be added in gitlab!59803 (merged))
• Code review guidelines
• Go Style guidelines

Related to #500 (closed)

/_redirects sample with new validations