[PHP] Add missing qualifier for enum parameters

[JulianVennen]

If you have an endpoint that takes a single instance of an enum as a parameter, its Qualifier (e.g. \OpenAPI\Client\Model) is missing in both the PHPDoc and the function parameter types. This PR fixes that. (See this method in the echo_api for an example).
This PR fixes that by adding an override for getParameterDataType in the AbstractPHPGenerator.

For some documentation files this causes a duplicated qualifier, but I am pretty sure this happens because these classes do not have a baseType (that is used in the generator when setting the example value). This can also be seen because the link to the documentation file for these models (in the table following it) is just ../Model/.md which is generated from ../Model/{{baseType}}.md.
For enum arrays and maps, this duplicated qualifier already exists before the changes I made.
I was not able to figure out what would cause this to be null/empty, but it seems unrelated to this PR.

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@jebentier, @dkarlovi, @mandrean, @jfastnacht, @ybelenko, @renepardon

[wing328]

I was not able to figure out what would cause this to be null/empty, but it seems unrelated to this PR.

can you please open a separate issue to track it when you've time?

[wing328]

tested locally and the result is good