Add the ability to optionally merge automatically inferred rules with manual rules

[CWAscend]

💪 Motivation

I would like the ability to use a combination of Attribute rules and manual rules that are defined within the static rules method in a Data object. For me, this will massively clean up my rules method when I have to reach for it, such as which I might be reaching for a callable rule, or using the Rule::in() rule where the array is dynamic - just to name some examples.

🛠 Changes

• Added a new Spatie\LaravelData\Attributes\MergeRules attribute
• This will merge the rules from the rules method rather than overwriting them
• Example usage can be found in the test Data class here: tests/Fakes/DataWithMergedRuleset.php

🧪 Testing

• A new test has been created: it will merge validation rules, which ensures both validation rules from the Attributes and the rules method are picked up.
• A new test has been created: it will merge validation rules using string rules which ensures the validation rules can be a piped string of rules instead of an array of files for each attribute

🧙 Reminders (Author)

• [x] Have you read through your own diff? 👀
• [x] Are all tests passing? 🧪