Create API endpoint to retrieve contract information on host

Created by: nielscastien

This PR introduces an API call to get basic information about stored contracts on the host.

Usage: curl -A "Sia-Agent" "localhost:9980/host/contracts?status=xxxx" where xxxx can be any of the following:

1. unresolved
2. succeeded
3. failed
4. rejected
5. all

First step to implement #2465 (closed)