feature/dynamodb/expression: support for composite keys

[raduendv]

Adds support for composite keys, necessary for using composite keys in Global Secondary Indexes.

Currently you cannot use the expression.Builder to create more complex KeyConditionss as the only available method is And() which results in an invalid state for the KeyCondition if you do something like this:

	builder := expression.NewBuilder()
	keyCondition := expression.Key("pk1").Equal(expression.Value("1"))
	keyCondition = keyCondition.And(expression.Key("pk2").Equal(expression.Value("1")))
	keyCondition = keyCondition.And(expression.Key("pk3").Equal(expression.Value("1")))
	keyCondition = keyCondition.And(expression.Key("pk4").Equal(expression.Value("1")))
	keyCondition = keyCondition.And(expression.Key("sk1").Equal(expression.Value("1")))
	keyCondition = keyCondition.And(expression.Key("sk2").Equal(expression.Value("1")))
	keyCondition = keyCondition.And(expression.Key("sk3").Equal(expression.Value("1")))
	keyCondition = keyCondition.And(expression.Key("sk4").Equal(expression.Value("1")))

	builder = builder.WithKeyCondition(keyCondition)
	expr, err := builder.Build() // err is: buildKeyCondition error: invalid key condition constructed
	// disabling the checks in `expression.KeyAnd()` would make `expr.KeyCondition()` something like `(((((((#0 = :0) AND (#1 = :1)) AND (#2 = :2)) AND (#3 = :3)) AND (#4 = :4)) AND (#5 = :5)) AND (#6 = :6)) AND (#7 = :7)`

New way:

	builder := expression.NewBuilder()
	keyCondition := expression.CompositeKey()
	keyCondition = keyCondition.AddPartitionKey(expression.Key("pk1").Equal(expression.Value("1")))
	keyCondition = keyCondition.AddPartitionKey(expression.Key("pk2").Equal(expression.Value("1")))
	keyCondition = keyCondition.AddPartitionKey(expression.Key("pk3").Equal(expression.Value("1")))
	keyCondition = keyCondition.AddPartitionKey(expression.Key("pk4").Equal(expression.Value("1")))
	keyCondition = keyCondition.AddSortKey(expression.Key("sk1").Equal(expression.Value("1")))
	keyCondition = keyCondition.AddSortKey(expression.Key("sk2").Equal(expression.Value("1")))
	keyCondition = keyCondition.AddSortKey(expression.Key("sk3").Equal(expression.Value("1")))
	keyCondition = keyCondition.AddSortKey(expression.Key("sk4").GreaterThanEqual(expression.Value("1")))

	builder = builder.WithKeyCondition(keyCondition)
	expr, err := builder.Build() // err is nil
	// `expr.KeyCondition()` is something like `((#0 = :0) AND (#1 = :1) AND (#2 = :2) AND (#3 = :3) AND (#4 = :4) AND (#5 = :5) AND (#6 = :6) AND (#7 >= :7))`