fix: prefer custom backoff strategy to built-in if provided

[lbennett-stacki]

This feels a bit easier to reason over to me, my thinking is...

1. A queue can define a built-in backoff strategy for use as an always-available default
   a. Queues no longer define their own custom strategies. Related to docs: delete page for now removed backoffStrategies Queue settings #2275 and fix: narrow the BackoffOptions "type" union #2276
2. A worker can define its own backoff strategy on that/any queue and the custom strategy will be preferred

Maybe a future piece could be to unify the backoff settings for both queues and workers. Or deciding one place to define it.

[roggervalf]

so this means that you want to have the possibility to override a builtin strategy? that's the only case that came into my mind, but it would be the same as defining a customFixed strategy or a customExponential one, also probably more people is using the default ones