feat(core): add conditional callbacks

[Archento]

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Based on feedback from the community and the opened discussion #249.

Adds conditional functionality to agents callbacks by providing the following capabilities without breaking backwards compatibility:

• add agent interface to get/set the current agent state as str
• add desired state as str to .on_interval() handler to only trigger that function based on the current agent state
• add desired state as str to .on_message() handler to only react to messages based on the current agent state
• decorators without the explicit mention of a state will act as usual

If desired we can add an example of state management