Allow configurable fallback tax calculator and tax rate calculator

[Noah-Silvera]

If a request to the taxjar api request fails due to Taxjar being unresponsive, we want provide users with the ability to set a fallback tax calculator and a fallback tax rate calculator that can be used in place of taxjar.

The user will be responsible for handling logging of what transactions passed through the calculator etc.

The calculator will only be used on failures that we can be sure represent taxjar failing to respond, as responding to all exceptions with a fallback calculator could go very wrong if the extension or consuming app has a bug which causes repeating failures.

This ticket arose out of the discussion in #60

[Noah-Silvera]

#61 must be completed before this ticket