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

Add filtering by Tx hash:

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

Alternatives Considered

We can create new endpoint, but we don't see any need for it.

Additional Context

Required for dashpay/platform#2261