retry: add core retry types and module structure

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>

crates/common/src/lib.rs

@@ -7,6 +7,7 @@ pub mod llm_providers;
 pub mod path;
 pub mod pii;
 pub mod ratelimit;
+pub mod retry;
 pub mod routing;
 pub mod stats;
 pub mod tokenizer;