feat: built-in retry policies (exponential backoff, exception-type filtering)

[thomhurst]

Description

RetryAttribute allows custom ShouldRetry logic, but there are no built-in patterns for common retry scenarios.

Missing Patterns

• Exponential backoff with jitter
• Retry only on specific exception types (transient failures)
• Timeout-specific retry policy
• Combined/composed retry policies

Proposed Approach

• Create IRetryPolicy interface with built-in implementations
• Add attributes like [ExponentialBackoffRetry(3, initialDelayMs: 100, multiplier: 2.0)]
• Create RetryContext with attempt number, delay, and exception info
• Add metrics for retry success/failure rates