add extended generic section

[celek]

End-user friendly description of the problem this fixes or functionality that this introduces

• [ ] Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

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Give a summary of what the PR does, explaining any non-trivial design decisions Creates a dynamic generic 'extended' section in config.toml

[extended]
key_1 = value_1
key_2 = value_2

you can access the value using:

value = cfg.extended.key_1

or

value = cfg.extended["key_2"]

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Link of any specific issues this addresses https://github.com/All-Hands-AI/OpenHands/issues/5849