Added new option methodNamingConvention to abstract php class

[web-mi]

This PR added new additionalProperty "methodNamingConvention" to handle special cases with mutiple properties like "test" and "_test" which will end in the same getter and setter "getTest". (e.g. Rocket.chat GetApiV1ImList200ResponseImsInner.php) By setting methodNamingConvenetion to snake_case the method names will be snake_case like "get__test" and "get_test" which is not the php default but solves the problem and makes the generated classes useable without a duplication error. Also if there are two properties in openapi.yaml like "o_auth_test" and "oauth_test" it will end in a duplicate getter like "getOAuthTest" and "getOauthTest". (e.g. Keycloak RealmRepresentation.php)

This will add a solution for #1633 Its an alternative for #1634 too.

Technical Commitee PHP: @jebentier, @dkarlovi, @mandrean, @jfastnacht, @ybelenko, @renepardon

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