Missing way to set rules for nested arrays easily

## Describe the bug

I'd like to validate something with structure like:
```
{
foos: [
{
bars: [
{some_entry: 1},
{some_entry: 2}
]
}
]
}
```

As far as I can tell, there's no good way to do that. I can't nest rules. There's a way to validate the nested entries with key paths, like in here: https://github.com/dry-rb/dry-validation/issues/732#issuecomment-1770474838
But this is getting really messy. At that point the rule is not really for `:foos` either - it's for something much deeper.

## Expected behavior

I'd really like to either stack contracts, so that the rules can be attached there, like:
```
class FooContract < Dry::Validation::Contract
params do
required(:bars).hash(BarContract)
end
rule(:some_entry) ...
end

class MainContract < Dry::Validation::Contract
params do
required(:foos).array { hash(FooContract) }
end
end
```

Or the ability to stack rules, so:
```
class MainContract < Dry::Validation::Contract
params do
required(:foos).array { hash(FooParams) }
end
rule(:foos).each do |foo|
rule(foos: foo).each do |bar|
key(....)....
end
end
end
```

## My environment

- Affects my production application: No