Add linear retry strategy with configurable increment

Add linear retry strategy for parity with JavaScript:
https://github.com/aws/aws-durable-execution-sdk-js/pull/513

```
Add createLinearRetryStrategy function with customizable initial delay and increment

Add linear preset to retryPresets for 1s, 2s, 3s, 4s, 5s delays

Export createLinearRetryStrategy from main SDK index

Include tests for linear backoff functionality
```