Extend the list of supported (chat) generators in langfuse for custom generators

[alex-stoica]

Is your feature request related to a problem? Please describe.
A custom component might have the correct output to allow for integrations/langfuse/src/haystack_integrations/tracing/langfuse/tracer.py to work. However, because the generators list is structured as follows:

_SUPPORTED_GENERATORS = [
    ...
]
_SUPPORTED_CHAT_GENERATORS = [
    ...
]
_ALL_SUPPORTED_GENERATORS = _SUPPORTED_GENERATORS + _SUPPORTED_CHAT_GENERATORS

it will not work with tracing unless the custom component is incorrectly placed into one of these predefined categories, which is not an ideal or fitting solution

Describe the solution you'd like
A function like register_generator_for_tracing could be created, with a parameter to specify whether the generator is a chat generator or not. This function would also check whether the output format meets a certain standard before registering the generator.