Add functions to generate Scram Credentials

[chatton]

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This PR moves the Credential generating logic from the enterprise code base to community. There are no functional changes. I've added additional unit tests for Sha256 secret generation.

[chatton]

test cases are using the salt, stored key and sever keys of users created through the mongo shell. E.g.

 db.createUser({user: "user4", 
pwd: "P8z1sDfELCePTNbVqX", 
roles: [ { role: "readWrite", db: "admin" } ]})

db.system.users.findOne({user: "user4"})

outputs

example-mongodb:PRIMARY> db.system.users.findOne({user: "user4"})
{
	"_id" : "admin.user4",
	"userId" : UUID("6b5aec1f-312b-491a-aeaa-b00cc374cf52"),
	"user" : "user4",
	"db" : "admin",
	"credentials" : {
		"SCRAM-SHA-1" : {
			"iterationCount" : 10000,
			"salt" : "Cd9RJHbdRt5b+kPf8YR0WQ==",
			"storedKey" : "v3BB0CLs5AwLiAgIfE/yqDujoI4=",
			"serverKey" : "yLqlYgg5GW04nAeBmFpgLU/vtto="
		},
		"SCRAM-SHA-256" : {
			"iterationCount" : 15000,
			"salt" : "RPNhenwTHlqW5OE597XpuwvPLaiecPpYFa58Pg==",
			"storedKey" : "sJ8UhQRszLNo15cOe62+HLjt2NxmSkJGjdJpclTIMBs=",
			"serverKey" : "CSg02ODAvh9+swUHoimXcDsT9lLp/A5IhQXavXl7+qA="
		}
	},
	"roles" : [
		{
			"role" : "readWrite",
			"db" : "admin"
		}
	]
}

[rodrigovalin]

just a small comment to get rid of one function indirection, otherwise LGTM

[rodrigovalin]

this function only dispatches the request to computeCreds and then, there's branching in computeCreds depending on that..

Why not:

func ComputeScramSha256Creds(username, password string, salt []byte) (ScramCreds, error) {
	base64EncodedSalt := base64.StdEncoding.EncodeToString(salt)
	return computeScramCredentials(sha256.New, scramSha256Iterations, base64EncodedSalt, password)
}

func ComputeScramSha1Creds(username, password string, salt []byte) ScramCreds, error) { 
	base64EncodedSalt := base64.StdEncoding.EncodeToString(salt)
	password = md5Hex(username + ":mongo:" + password)
	return computeScramCredentials(sha1.New, scramSha1Iterations, base64EncodedSalt, password)
}

[chatton]

this is actually much nicer, as the Sha256 function doesn't actually require username 👍