Enhancement: lookup by fqn command

[jakedoublev]

Background

Currently in the platform there are 4 types of Fully Qualified Names (FQNs) that correspond to objects within OpenTDF Platform policy:

1. attribute namespaces: https://<namespace>
2. attribute definitions: https://<namespace>/attr/<attribute definition name>
3. attribute values: https://<namespace>/attr/<attribute definition name>/value/<value>
4. resource mapping groups: https://<namespace>/resm/<group name>

Two things are possible and maybe even likely:

1. the number of objects that can be fully qualified with a namespace will grow (registered KASes, various mappings)
2. the structure of the FQN itself may change

The otdfctl CLI has done a great job simplifying the CRUD of policy (attributes/mappings) for platform administrators. However, as the data is truly a graph structure, there is a known desire to understand policy objects by their names rather than their IDs.

There are already some service-to-service RPCs that exist within the platform to contextualize various FQNd policy objects without use of UUIDs:

1. GetAttributesByValueFQNs (with lookup for a value-less attribute FQN already existing within)
2. ListResourceMappingsByGroupFQNs

The behavior of these existing RPCs is different from our normal GET/LIST behaviors because it is known that requests to these RPCs seek great depth of context around the policy object being queried. They are service-first RPCs rather than admin-CRUD-first RPCs. That said, we can expose them in the CLI via a lookup functionality. This would be durable to any future FQN extensions or changes and serve the admin/PEP developer need to understand everything relevant to a policy object.

Acceptance Criteria

1. otdfctl policy lookup [fqn] takes any supported FQN as an argument
2. the CLI breaks down the FQN argument into its known supported kind
3. the CLI calls the relevant Policy FQN-driven-read RPC to provide the entire context (all mappings, all grants, etc) to the policy object
4. the CLI outputs the JSON response