jellyfish-block to support creating, signing and submitting a block from scratch

[fuxingloh]

What would you like to be added:

Either within packages/jellyfish-block or a new package altogether, we should support:

1. Creating an Unsigned Block (with different scenarios)
   1. With only Coinbase Transactions
   2. With UTXO-only transactions
   3. With DfTx account-level transactions
2. Signing the given Block
3. Submitting the signed Block via jellyfish-api-core

More RPC from jellyfish-api-core can be used to aid the process. Otherwise, it's preferred that we rely 100% on jellyfish-* JS packages.

I Highly recommend this issue be completed in small parts due to its complexity.

Why is this needed:

/assign @DocteurPing as a continuation of #48 — submitblock and getblocktemplate, by further venturing deep into how a block gets created and supporting that functionality on the jellyfish core.
/triage accepted
/area packages