feat: Tower based Middlewares

[64bit]

• add support for middleware with Tower Service and Layer abstractions
• add OpenAIRetryLayer which is specific for retrying OpenAI API, there's more generic retry policy SimpleRetryPolicy, both implements exponential backoff. This also removes the backoff crate dependency, and enables retries in WASM.
• Add examples/{tower,middleware,tower-wasm}
• Add new middlware docs in MIDDLEWARE.md doc and lib.rs
• All is behind middleware feature flag