Support custom retry strategies

[maxscheijen]

Description:

Allow users to plug in their own retry/backoff strategies beyond exponential.

For example:

• Fibonacci backoff
• Linear backoff
• Custom callable for delay calculation

Acceptance Criteria:

• Accepts a strategy parameter or callable to compute next delay.
• Includes at least one built-in alternative (e.g., Fibonacci).
• Document how to implement custom strategies.