perf(aws): aws signer has cache and can be reused

[sjiekak]

aws v4.Signer is implemented for reuse

What is meant to be saved is recomputing the key (which require 4 HMACSHA256) calls.

// DeriveKey returns a derived signing key from the given credentials to be used with SigV4 signing.
func (k *SigningKeyDeriver) DeriveKey(credential aws.Credentials, service, region string, signingTime SigningTime) []byte {
	return k.cache.Get(credential, service, region, signingTime)
}

func deriveKey(secret, service, region string, t SigningTime) []byte {
	hmacDate := HMACSHA256([]byte("AWS4"+secret), []byte(t.ShortTimeFormat()))
	hmacRegion := HMACSHA256(hmacDate, []byte(region))
	hmacService := HMACSHA256(hmacRegion, []byte(service))
	return HMACSHA256(hmacService, []byte("aws4_request"))
}