RetryGatewayFilterFactory should support custom exchange attribute

Describe the bug
The current implementation uses RETRY_ITERATION_KEY="retry_iteration" as the constant attribute name for tracking the iteration in the exchange. This prevents leveraging different retry strategies for different cases - for example, for IOExceptions we want to retry up to 3 times, but for authentication issues using cached credentials we want to clear the cache, and try again 1 time.

There should be a default name, but it should be configurable which name to actually use.

It may be possible to work-around this by introducing intermediate filters which "copy" that attribute to some other attribute and vice-versa as part of the outgoing and returning chains. Alternatively, a decorator could be applied to the exchange in a preceding filter such that the RETRY_ITERATION_KEY is mapped to some other attribute key.

Start by reading RetryGatewayFilterFactory and how it stores RETRY_ITERATION_KEY on the exchange. Trace the retry configuration and related tests, then define what a configurable attribute name with the existing default should affect. Done means separate retry strategies can use distinct exchange attributes without breaking the default behavior.