Add type hints to KlaviyoAPI via PHPDoc

[AJenbo]

Using the API is not very friendly as it does not hint the types and so you constantly have to look things up either in code or docs since the IDE is not able to reason about the code. Also the lack of types prevents static analyzers from validating that the code has been written correctly.

I have added some basic hints to the main class, but there appears to be more room for improvements.

[chrishalbert]

Since this sdk supports 7.4+, we could use type declarations when defining the properties and method signatures. That would add an extra layer of defensive coding around the public properties, which are otherwise open for modification.

The only issue that I can see is that existing code which relies on the dynamic typing of php could break, and with breaking changes comes another major version tag. Then again, this would ensure that the remainder of the code functions exactly as expected though.

[AJenbo]

Yeah I wanted to avoid any breaking changes, but also we need generics to express how the Subclient works which can only be done via PHPDoc as of now.

[chrishalbert]

Oh yes, I recognized that syntax - I've spent more time trying to get that passing phpstan than doing the actual fix :P Sounds good and in that case, I approve

[obennaci]

Any chance to have this merged? it's a great QoL change 🙏