Chain simulator custom transactions sender component

[iulianpascalau]

Reasoning behind the pull request

• the chain simulator uses the same component used by the regular node to broadcast the transactions. The default component contains an accumulator that gathers transactions for about 100 milliseconds and then dispatches them as a group of transactions. The dispatching mechanism uses a data packer to split the accumulated transactions into smaller packets (if required and if it is possible)
• renaming the keys to pairs (related to Rename "keys" into "pairs" #6179):
  The endpoint /address/:address?withKeys=true is going to return something of this type:

type AccountResponse struct {
	Address         string            `json:"address"`
	Nonce           uint64            `json:"nonce"`
	Balance         string            `json:"balance"`
	Username        string            `json:"username"`
	Code            string            `json:"code"`
	CodeHash        []byte            `json:"codeHash"`
	RootHash        []byte            `json:"rootHash"`
	CodeMetadata    []byte            `json:"codeMetadata"`
	DeveloperReward string            `json:"developerReward"`
	OwnerAddress    string            `json:"ownerAddress"`
	Pairs           map[string]string `json:"pairs,omitempty"`
}

Merged PR: https://github.com/multiversx/mx-chain-core-go/pull/298/files

For the sake of coherence, it would be best if the chain simulator uses pairs term instead of keys.

Proposed changes

• add a new transactions send handler that does not accumulate transactions but rather sends them directly, without delay to the network messenger. This component should be used only in the chain simulator, hence the definition and implementation in the chainSimulator package.

Testing procedure

• standard system test (no production code involving the node was touched)
• chain simulator tests should pass

Pre-requisites

Based on the Contributing Guidelines the PR author and the reviewers must check the following requirements are met:

• was the PR targeted to the correct branch?
• if this is a larger feature that probably needs more than one PR, is there a feat branch created?
• if this is a feat branch merging, do all satellite projects have a proper tag inside go.mod?