Add option to keep JSON readable for some implementations

[philippgille]

Currently, when JSON is used as MarshalFormat, it will always be converted to []byte. This makes the result unreadable as a human, unless interpreted as / converted back to string. So for example when using DynamoDB, and a struct is marshalled into a JSON {"Foo":"bar"}, saved to DynamoDB with gokv, and then you look at the value via the AWS console, it's just eyJGb28iOiJiYXIifQ==, the Base64 encoding of the JSON string.

That's because in the dynamodb implementation we use a awsdynamodb.AttributeValue for the value where we assign the value to a field B, which is for []byte, and has the Base64 encoding described in the comment:

// An attribute of type Binary. For example:
//
// "B": "dGhpcyB0ZXh0IGlzIGJhc2U2NC1lbmNvZGVk"
//
// B is automatically base64 encoded/decoded by the SDK.
B []byte `type:"blob"`

In this case, we could look at the option provided by the package user and instead of B, use S and assign the plain JSON string to it.

This should also be done in a similar way for other implementations. It only makes sense for some of them though!