Implement Project-level defaults

As part of RFD 21 and the discussion on #1003, we'd like to implement defaults at the level of a Project. The canonical example from RFD 21 is a default VPC Subnet, in which we create NICs for a guest if one is not explicitly provided in the client request. Defaults should be configurable resources, at the Project scope, for example via CRUD on an endpoint like /organizations/{org}/projects/{project}/defaults. We'll need a new table in CRDB to manage these. Below is a preliminary list of the defaults we'd like to include, but more should be added as they make sense.

• VPC
• VPC Subnet
• Image from which guest instances are created
• Whether guest NICs default to dual IPv4/v6, or exactly one of those. As an example, if the default is to provide both versions and the client requests only a specific IPv4 address, an IPv6 address is added automatically.

Start by reading RFD 21 and the discussion on issue #1003 to establish the intended Project-level defaults model. Then inspect the existing Project-scoped CRUD endpoints and CRDB schema patterns. Done means agreeing on the supported defaults, exposing their CRUD endpoint, and persisting them in a new table.