feat: add decorators for mellea tools

[jakelorocco]

PR #380 added MelleaTools as a way to pass tools to models (removing our previous approach of using callables).

This adds some cumbersome / clunky code in places.
Previously:

tools = [get_weather, search, ...]

Now:

tools = [MelleaTool.from_callable(get_weather), MelleaTool.from_callable(search)]

We should add a mellea_tool or tool decorator (naming not important) for functions so that you can just reference them by their name if you plan to only use them as tools.

[akihikokuroda]

May I take this?