vmod_blob: Add convenience functions for urldecode and urlencode

[nigoroll]

For the common cases, the generic and versatile syntax is really cumbersome.

Why exactly did we not add these earlier?

[simonvik]

Could it be worth writing somewhere that blob.urldecode might be unsafe and truncate values if the url contains %00? (if thats the case)

[nigoroll]

@simonvik good point.
Are there any established alternatives to handling %00 like throwing an error?
If yes, I think we should rather review any IDENTITY encoding to STRING.

[nigoroll]

This triggered #4255