tECDSA signing: Cleanup `node.go`

[lukasz-zimnoch]

Refs: https://github.com/keep-network/keep-core/issues/3042 Depends on: https://github.com/keep-network/keep-core/pull/3201

Since we want to implement signing as the next feature of tbtc.node, we need to make cleanup of the pkg/tbtc/node.go file. We do it here by extracting all DKG components to a separate file pkg/tbtc/dkg.go. Worth noting that the tbtc.node.joinDKGIfEligible function remains unchanged and is still responsible for the orchestration of the aforementioned components. I made some experiments trying to split it as well but that yields an additional level of indirection resulting with a less readable code.

By the way, this PR addresses two TODOs from #3120 regarding the GroupQuorum parameter: https://github.com/keep-network/keep-core/pull/3209/commits/83eb9f02bd2056a208e8e028cb16d7808cccabf6