Define custom resource request schema

There's no place to request some types of resources:
- GPUs (architecture, brand, model, memory, number of cores?)
- Disk type
- CPU architecture
- AWS Spot Market bidding strategy
- Cloud/cluster-specific resources such as specialized hardware (e.g. Tensor Processing Unit)

Also, some existing concepts in the `Resources` schema are not clear
- zones: not a consistent concept across clusters, e.g. in GCE there are multiple global regions, each with multiple zones.
- `preemptible` does not fully capture the concept in AWS

A proposed solution is to remove some concepts from the schema, such as `preemptible` and `zones`, and add a new field of type `map[string]string` to allow more flexible, implementation-specific resource requests.

Start by reviewing the existing Resources schema and the discussion in issue #84, focusing on GPU, disk, CPU architecture, spot bidding, specialized resources, zones, and preemptible requests. Done means the project has agreed on and documented a concrete schema design that addresses these resource types and replaces or revises the unclear concepts.

Written by the indexing model from the issue text.