This change allows a user to specify `contextCache` method in the
`experimental_nextCacheLink`, which lets one select the values
from the context that should be considered when a `cacheTag` is
generated.

It also changes the way `cacheTag` is generated, taking care of
the fact that the `cacheTag` cannot be longer then 256 characters,
thus instead of stringifying input values, it will form a sha-256 hash
of those.

Implements trpc#5455

…er creation