Make the retry middleware more flexible

[xico42]

Motivation / Background

This PR addresses the issue #571. The idea here is to allow clients to specify exactly which errors are retryable or not.

Retryable errors are typically caused by transient failures such as timeouts or connection issues.

However, there may be cases where certain errors are known to be non-retryable, for example, malformed input messages.

This change adds a ShouldRetry parameter to the retry middleware configuration, allowing clients to explicitly define which errors should be retried and which should not.

Detail

At the company I work for, we use a custom error package that allows us to have a fine control of which errors are retryable or not.

So, it would be great if the Retry middleware could be customized to ignore those errors we already know
that are not retryable.

Alternative approaches considered (if applicable)

Not considered!

Checklist

• I wrote tests for the changes.
• All tests are passing.
• Code has no breaking changes.
• Documentation on watermill.io is updated.
  • I have updated only the godoc documentation