Models for ledger state objects

[mDuo13]

We should have models for ledger data, including the ledger header and all types of ledger objects that can be in the ledger's state data.

This would aid reading/exploring and processing ledger data as well as transaction metadata.

[intelliot]

More important than other enhancements, but not truly a bug fix

[intelliot]

More important than adding response objects

[intelliot]

May make sense to prioritize if/when devs request it