Add some reference material on golang Context

[illicitonion]

Both of our Cohort 2 trainees have raised existential questions around Context. We may want to find some useful references or write up some material about this.

We may even want to make a "implement WithTimeout using WithValue" exercise to show that there's no magic here, it's really just a fancy key-value store.

(I've also gone on a tangent around persistent data structures when talking through this, unclear whether that's actually useful/worthwhile :))

[illicitonion]

The journey I've typically used to explain this is:

• We know about dependency injection, when people take dependency injection to extremes they end up with too many parameters, they often extract a context object to contain them all (prompt discussion about global state, about the down-sides of God objects, ...).
• Context is a place to shove global-like data that we want to use a lot of places. But we generally shouldn't.
• Introduce With-prefixed functions (and maybe contrast with other persistent data structures / builders / ...)
• Talk through WithValue with a user ID as an example
• Talk through WithTimeout and Done as just a library written using WithValue.

@bazmurphy had also encountered React's Context hooks which implement a similar pattern.

[lauranooooo]

that is a good thought, IMO.
I am sure we must be using timeout somewhere. we certainly have material on the need for timeouts in the primer, don't we?
i think we could probably weave this into at least one of the exercises. maybe with some explicit demos of client cancellation as well, and a description of why that's useful.