googleai: fix tool conversion in case of complex schema

[rainu]

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I figured out that tool conversation did not work in case of complex tool definitions. Especially for arrays and inner-objects. So in this PR I refactored the tool-conversation logic and wrote some tests (only for the conversation logic). Furthermore, I implements the possibility to "outsource" the schema-definition to the user of this library. This is the case if the user will set the schema directly as Tool's Parameter.