Support plugins

[rianhughes]

This PR implements plugin support. Any plugin that follows the JunoPlugin interface can plug into Juno.

type JunoPlugin interface {
	Init() error
	Shutdown() error // Todo: Currently this function will never be called.
	NewBlock(block *core.Block, stateUpdate *core.StateUpdate, newClasses map[felt.Felt]core.Class) error
	// The state is reverted by applying a write operation with the reverseStateDiff's StorageDiffs, Nonces, and ReplacedClasses,
	// and a delete option with its DeclaredV0Classes, DeclaredV1Classes, and ReplacedClasses.
	RevertBlock(from, to *BlockAndStateUpdate, reverseStateDiff *core.StateDiff) error
}

Whenever Juno syncs a new block, the block, state-update and classes are sent to the plugin. Specifically, NewBlock() is called
NewBlock(block *core.Block, stateUpdate *core.StateUpdate, newClasses map[felt.Felt]core.Class) error

Whenver Juno reverts a block, it will send the block and state-updateof the current height "N", the reverted height "N-1", as well as the "reverse state diff" that brings you from N to N-1. Specifically, RevertBlock() is called
RevertBlock(from, to *BlockAndStateUpdate, reverseStateDiff *core.StateDiff) error

Note: the code coverage is below the threshold. The main chunk of code that hasn't been tested is the Load() function. A simple plugin that only prints when a function has been called can be quite large (~37Mb+), so it doesn't seem feasible to include a dummy plugin/.so file for the purpose of increasing the code coverage. However, the Load function was tested manually and it seems to work just fine (see here for example plugin)

[codecov]

Codecov Report

Attention: Patch coverage is 45.09804% with 56 lines in your changes missing coverage. Please review.

Project coverage is 78.26%. Comparing base (9a3f263) to head (e387614).
Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
sync/sync.go	51.21%	12 Missing and 8 partials ⚠️
node/node.go	15.38%	10 Missing and 1 partial ⚠️
plugin/plugin.go	0.00%	11 Missing ⚠️
core/state.go	58.33%	6 Missing and 4 partials ⚠️
blockchain/blockchain.go	66.66%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2051      +/-   ##
==========================================
- Coverage   78.45%   78.26%   -0.19%     
==========================================
  Files         100      103       +3     
  Lines        9240     9294      +54     
==========================================
+ Hits         7249     7274      +25     
- Misses       1353     1377      +24     
- Partials      638      643       +5     

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[weiihann]

I'd suggest to add some documentations so that users are aware of this feature