* Remove helper func getCommitmentAddress.
This function is only used in one place so removing it does not introduce any duplication. Removing the func also removes the need for errors.Is comparison, which will be very useful in upcoming changes.
* Rename apiError to ErrorCode.
* Don't use inline type for API errors.
* Export webapi errors.
* Export webapi request/responses.
* Add types module for API requests/responses/errors
`CanTicketVote` doesn't really fit into the RPC client code as it is more of a business logic function. Moving it into the webapi package is more appropriate.
This allows both tspend and treasury policies to be set by clients on a per-ticket basis. Preferences can be set when initially registering a ticket with `/payfee`, and can be later updated using `/setvotechoices`.
Any requests which alter treasury/tspend policy will be stored in the database using the existing accountability system.
**Note:** This does not include consistency checking, it will need to be added later when dcrwallet has an RPC to retrieve policies in batches.
* Rework client/server authentication.
- Remove Signature from all requests, and instead expect a signature in HTTP header "VSP-Client-Signature".
- Remove CommitmentSignatures from the database.
- Use a bool flag to indicate when a ticket is missing from the database rather than an error.
This commit introduces a lot of duplication into each of the authenticated HTTP handlers. This should be removed in future work which moves the authentication to a dedicated middleware.
* Introduce auth and rpc middleware.
This removed the duplication added in the previous commit, and also removes the duplication of RPC client error handling.
