feat(rpc): filter unconfirmed txs by tx hash

[lklimek]

Expected Behavior

We want to check if one particular tx is still in a mempool

Current Behavior

We can only retrieve all txs in mempool (paginated)

Possible Solution

We can create new endpoint to fetch unconfirmed tx by hash.

Alternatives Considered

Add filtering to RequestUnconfirmedTxsby Tx hash:

type RequestUnconfirmedTxs struct {
	Page    *Int64          `json:"page"`
	PerPage *Int64          `json:"per_page"`
	TxHash    *bytes.HexBytes `json:"tx_hash"`
}

Additional Context

Required for dashpay/platform#2261