roachtest: refactor ClusterSpec to allow easier reporting of configs

[renatolabs]

ClusterSpec[1] is the struct that defines the infrastructure where a roachtest will run. Some fields in that struct are defined via a command line flag (e.g., cloud) and never changed again, while others may or may not be specified by tests and are resolved later when left blank (e.g., InstanceType, whether to use ZFS, encryption-at-rest, etc).

When a value is not specified and is resolved later, it is often not easy to find the value used. For instance, tests can opt-in to randomly use ZFS (RandomlyUseZfs). When enabled, there's a 20% chance of running with ZFS [2]; however, there's no easy way to report whether ZFS was used in a test failure because that value is not exported in a way that is easy to consume by other callers (most notably, when reporting parameters for a test failure [3]). This same happens for instance/machine type [4] and encryption-at-rest [5].

Ideally, we want to be able to have all these computed values easily accessible to higher layers in the stack when we report test failures.

Related issues and pull requests: #80799, #81845.

[1] https://github.com/cockroachdb/cockroach/blob/467a86fced31ef7c011f3bb8ab47499c6a6f497b/pkg/cmd/roachtest/spec/cluster_spec.go#L39-L62?plain=1
[2] https://github.com/cockroachdb/cockroach/blob/467a86fced31ef7c011f3bb8ab47499c6a6f497b/pkg/cmd/roachtest/spec/cluster_spec.go#L225-L230?plain=1
[3] https://github.com/cockroachdb/cockroach/blob/467a86fced31ef7c011f3bb8ab47499c6a6f497b/pkg/cmd/internal/issues/formatter_unit.go#L76?plain=1
[4] https://github.com/cockroachdb/cockroach/blob/467a86fced31ef7c011f3bb8ab47499c6a6f497b/pkg/cmd/roachtest/spec/cluster_spec.go#L192-L200?plain=1
[5] https://github.com/cockroachdb/cockroach/blob/467a86fced31ef7c011f3bb8ab47499c6a6f497b/pkg/cmd/roachtest/test_runner.go#L606-L616?plain=1

Jira issue: CRDB-16093