Refactor chain manager subnets

[joshua-kim]

Why this should be merged

Needed as a pre-requisite to remove vm.Initialize. This refactors the currently running subnets into its own struct, so it can be shared with a future chains.Factory struct which will be responsible for creating chain instances (currently this is created internally in the chain manager). Ideally we'll have a chain factory to create chains and a chain manager that holds the chains that are currently running. This way, the chain manager doesn't need to know about the p-chain at all since the caller can choose arguments passed into the chain factory instead and the p-chain logic can be in the caller instead of in chains/manager

How this works

Refactors currently running subnets into its own struct

How this was tested

Added unit tests

[aaronbuchwald]

LGTM with optional nits

[StephenButtolph]

Running locally resulted in:

[02-10|13:01:49.389] INFO node/node.go:899 initializing chains
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x0 pc=0x1028a12c8]

goroutine 1 [running]:
github.com/ava-labs/avalanchego/chains.(*Subnets).Get(0x0?, {0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...})
        /Users/stephen/go/src/github.com/ava-labs/avalanchego/chains/subnets.go:50 +0x28
github.com/ava-labs/avalanchego/chains.(*manager).StartChainCreator(0x14002a64380, {{0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...}, ...})
        /Users/stephen/go/src/github.com/ava-labs/avalanchego/chains/manager.go:1337 +0x4c
github.com/ava-labs/avalanchego/node.(*Node).initChains(0x1400024b200, {0x1400175f800, 0x1bc2, 0x3800})
        /Users/stephen/go/src/github.com/ava-labs/avalanchego/node/node.go:910 +0x12c
github.com/ava-labs/avalanchego/node.New(0x140000d7600, {0x1039e9b18, 0x14000362380}, {0x1039f0320, 0x14000398080})
        /Users/stephen/go/src/github.com/ava-labs/avalanchego/node/node.go:261 +0x14e4
github.com/ava-labs/avalanchego/app.New({{{0x6fc23ac00, 0x6fc23ac00, 0x6fc23ac00, 0x1bf08eb000}, {{0x0, {...}}, {0x0, 0x0}, {0x0, {...}, ...}, ...}, ...}, ...})
        /Users/stephen/go/src/github.com/ava-labs/avalanchego/app/app.go:73 +0x3a0
main.main()
        /Users/stephen/go/src/github.com/ava-labs/avalanchego/main/main.go:48 +0x2cc

(which is presumably why all the E2E tests are failing).