Allow updating `ConnContext`

[ShivanshVij]

The ConnContext function allows developers to set a function that will be called on new connections and set a context for them that will be passed to each request.

It makes sense to offer an API within Frisbee that allows someone to update this context from within handler functions.

This function is a requirement for the management rewrite because it allows us to verify information about a connected agent, and save that information in a context that will be available for all future handler calls.