Add support for dynamic tools in sessions

[creatorrr]

That is, people should be able to send tools in the session itself in addition to the tools of the agent.

So basically allow POST {tools, ...} to /session/chat and merge these with agent.tools

(More details pending; please message one of the maintainers if interested)
Maintainers: @alt-glitch @whiterabbit1983 @creatorrr