Local evaluation of script execution cost

[nielstron]

In order to properly estimate fees for transactions (fast), PyCardano should be able to estimate them entirely locally. This includes running the Plutus script in a local VM.

For this we need essentially two more features

• translating the txbuilder context to a script context plutus object to call the involved script (Add script context builder OpShin/opshin#92)
• a proper cost estimating implementation of the CEK machine in the python UPLC package (https://github.com/OpShin/uplc)