Support for decoding TX auxiliary data

[agaffney]

There are different possible auxiliary data formats for different eras, and we need to be able to support them all in later eras. We'll also need a common interface type for querying data from it no matter the format.

From the Conway CDDL:

auxiliary_data = 
  shelley_auxiliary_data/ shelley_ma_auxiliary_data/ alonzo_auxiliary_data

shelley_auxiliary_data = metadata

metadata = {* transaction_metadatum_label => transaction_metadatum}

transaction_metadatum_label = uint .size 8

transaction_metadatum = 
  {* transaction_metadatum => transaction_metadatum}
  / [* transaction_metadatum]
  / int                          
  / bytes .size (0 .. 64)     
  / text .size (0 .. 64)      

shelley_ma_auxiliary_data = 
  [transaction_metadata : metadata, auxiliary_scripts : [* native_script]]

alonzo_auxiliary_data = 
  #6.259(
    { ? 0 : metadata                
    , ? 1 : [* native_script]   
    , ? 2 : [* plutus_v1_script]
    , ? 3 : [* plutus_v2_script]
    , ? 4 : [* plutus_v3_script]
    }

  )