planner: add governance voting

[TalDerei]

There's a governance voting regression in the planner per #4300.

This issue attempts to describe how validator voting worked in the original planner, and the necessary steps required to fold that functionality back in.

1. The planner tracks voting intents in a BTreeMap, populated on calls to delegator_vote() which appends the user's weighted vote to the BTreeMap. This doesn't add the votes to the planner quite yet.
2. Query for notes to be used for voting on a proposal from the view service. For each voting intent, issue a NotesForVotingRequest from a specific starting block height to request the relevant voting note.
3. After collecting the voting spendable note records, we check if the delegation token (delegations to a validator are represented by a single delegation token, representing a share of that validator’s delegation pool) is unspent, and then add the voting SNR as a SpendPlan.
4. Call delegator_vote_precise() to add the votes to the planner as DelegatorVotePlan.

here's a reference for a first pass at an attempted voting fix.

cc @erwanor