Add a way to process routes before rendering

[SanderElias]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/scullyio/scully/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Other... Please describe:

What is the current behavior?

There is no way to alter the handledRoutes array

Issue Number: closes #700

What is the new behavior?

It exposes a new type plugin processRoutes, that can be used to make changes to the handledRoutes array.
the plugin receives, and should return an HandeledRoute[]. No rendering will take place before those plugins are done.
When you register a processRoute plugin you can give it a priority, the lowest priority gets called first. Defaults to 100

Does this PR introduce a breaking change?

• Yes
• No

Other information