Track the provenance of UTXOs

[hdevalence]

Is your feature request related to a problem? Please describe.

One of the transaction consensus rules, whose previous implementation was removed in f607c98 , requires that transactions with transparent inputs which are the outputs of coinbase transactions must have no transparent outputs. (The previous implementation did not actually check this).

This requires tracking the provenance of UTXOs to some extent, which may require changing the data we store in our database. It might be worth checking what Zcashd does here.

Describe the solution you'd like

Rather than just tracking raw transparent::Outputs, we should track Utxos, a new structure that includes a transparent::Output as well as metadata:

pub struct Utxo {
    pub output: transparent::Output,
    pub height: block::Height,
    pub from_coinbase: bool,
}

This requires:

• defining the Utxo structure;
• defining a serialization format for the Utxo structure as stored in the database;
• incrementing the database format version;
• changing the zebra_state::Response::Utxo variant to return a Utxo rather than a raw Output;
• correcting all previous uses of the UTXO code.

Then, in subsequent PRs, we can use this information to implement the relevant consensus rules.

[hdevalence]

Here's the relevant structure in Zcashd (thanks @str4d for the pointer): https://github.com/zcash/zcash/blob/877212414ad40e37cf8e49884a90767c54d59ba2/src/coins.h#L23-L79