async backing: fetch prospective parachains config from the Runtime

[slumber]

polkadot/node/core/prospective-parachains/src/lib.rs

Lines 58 to 68 in 6138103

	// The maximum depth the subsystem will allow. 'depth' is defined as the
	// amount of blocks between the para head in a relay-chain block's state
	// and a candidate with a particular relay-parent.
	//
	// This value is chosen mostly for reasons of resource-limitation.
	// Without it, a malicious validator group could create arbitrarily long,
	// useless prospective parachains and DoS honest nodes.
	const MAX_DEPTH: usize = 4;
	// The maximum ancestry we support.
	const MAX_ANCESTRY: usize = 5;

These values should be in sync with the Runtime.

Related #4841

[rphmeier]

I think the biggest question is whether we want these things to vary on a per-block basis (or even per-chain) or per-session. The easiest thing for the moment would be per-session: bake it into the HostConfiguration and have the same values for every chain.