Add the retry module with core type definitions including:
- RequestContext, RequestSignature for request deduplication
- RetryExhaustedError, AllProvidersExhaustedError for error handling
- AttemptError, AttemptErrorType for attempt tracking
- ValidationError, ValidationWarning for config validation
- Helper functions for provider extraction and hashing
Wire up pub mod retry in lib.rs.
Signed-off-by: Troy Mitchell <i@troy-y.org>
