Support getFeeStats in the RPC client (rpc.Server)

[Shaptic]

The getFeeStats method was introduced in stellar/stellar-rpc#172 with the following response schema:

interface Response {
	sorobanInclusionFee: FeeDistribution;
	inclusionFee: FeeDistribution;
	latestLedger: number; // uint32
}

interface FeeDistribution {
	max: string; 	// uint64
	min: string; 	// uint64
	mode: string; 	// uint64
	p10: string; 	// uint64
	p20: string; 	// uint64
	p30: string; 	// uint64
	p40: string; 	// uint64
	p50: string; 	// uint64
	p60: string; 	// uint64
	p70: string; 	// uint64
	p80: string; 	// uint64
	p90: string; 	// uint64
	p95: string; 	// uint64
	p99: string; 	// uint64
	transactionCount: string; // uint32
	ledgerCount: number; // uint32
}

Note that the pNN values are just string-ified 64-bit integers.