feat: add MaxRetries and MaxSleep options to customize retry behavior

[AdamMagued]

Description

This change introduces MaxRetries and MaxSleep functional options to allow callers to configure retry behavior when checking for goroutine leaks in Find, VerifyNone, and VerifyTestMain.

Previously, the maximum number of retry attempts (_defaultRetries = 20) and maximum sleep interval (100 * time.Millisecond) were hardcoded in buildOpts. Callers could not adjust these parameters for scenarios where goroutines require additional time to terminate, nor could they disable retries completely (e.g., via MaxRetries(0)) when testing code that must not leak goroutines even transiently.

Changes

• Added MaxRetries(n int) Option to customize the maximum number of retry attempts (defaulting to 20). Setting MaxRetries(0) terminates after the initial inspection without retrying.
• Added MaxSleep(d time.Duration) Option to customize the maximum exponential backoff sleep duration between retry attempts (defaulting to 100ms).
• Exported MaxSleep while preserving internal maxSleep compatibility.
• Added safety bounding in opts.retry before evaluating bit shifts to prevent integer overflow when large retry counts are configured.
• Added test coverage in options_test.go and leaks_test.go verifying that MaxRetries and MaxSleep configure option values, control retry termination, and adjust backoff timings.

Fixes #93

[AdamMagued]

Closing to align with previous maintainer direction on RetryPolicy abstraction.