Cache key policies

[cicdw]

This PR introduces the final user-facing feature in the transactions story: cache policies (formerly known as both idempotency key policies and transaction key policies).

At its core, the idea with cache policies is relatively simple: they expose configuration for how your task should cache its results across executions. In this spirit, this PR provides a menu of options for configuring this behavior. Some examples include:

• NONE: never cache
• INPUTS: cache based on a hash of the inputs to the task
• TASKDEF: cache based on the source code of the task function
• providing a cache_key_fn: in a future PR this will create a Custom policy definition

This gets interesting in two respects: first off, these policies can be manipulated through basic operations to create more advanced policies:

from prefect.records.cache_policies import INPUTS, TASKDEF

# a policy that hashes all inputs _except_ any input named "foo"
my_policy = INPUTS - "foo" 

# a policy that uses both hashed inputs _as well as_ task definition 
another_policy = INPUTS + TASKDEF 

At a deeper level, these policies configure the transactional behavior of tasks by specifying the location of the transaction record. These records are only written whenever a task commits itself.

cc: @EmilRex

[chrisguidry]

I dig it! It's a nice interface, just have a think about whether we want those add/subtract operations to be idempotent

[chrisguidry]

🔥 Awesome API