Use a hierarchical config format

We could switch from Python's config INI format to YAML (or JSON), enabling easy nesting of options.

For example:
```
client: qpu
solver:
num_active_qubits:
gt: 4000
topology:
type: pegasus
```

This would enable a partial update (`PATCH` with a diff), e.g:
```
solver:
topology:
type: chimera
```

Another possible syntax (closer to the current INI format, is [TOML](https://toml.io/)):
```
[defaults]
token = "..."

[qpu-chimera]
solver.topology.type = "chimera"

[dqm-nonbulk.solver]
name__regex = ".*(?