Allow key value based transformers with custom replacement function

[dfangl]

Motivation

Currently our KeyValueBasedTransformer only allows static values for replacement.
However, this limits us in the kind of transformers we can build - for me, I wanted to build a key value based transformer where the length of the value was part of the transformation.

Changes

• Allow replacement function instead of static value and rename to KeyValueBasedTransformerFunctionReplacement (a bit bulky)
• Create a new KeyValueBasedTransformer with a simple function returning the static value for compatibility with existing code (and convenience)

Open questions

• The reference replacement is based on the replacement value - so different length replacement within the same snapshot session will have different counters - they are still identifiable, but we do have two ":1" replacements for the same transformer, if the generated replacement differs. I think this is not a big problem, but maybe something we do not want.
• In general, we can ask ourselves the question if a transformer asserting the length is really something we need (however I argue the more accurate the better).