Add TTLMiddleware for automatic document expiration (#599)

[Anuradha-Agrawal-07]

This pull request introduces TTLMiddleware, a new middleware for TinyDB that enables automatic expiration of documents based on TTL (time-to-live) semantics. Documents can specify expiration using either:

_ttl: number of seconds before expiration from insertion time,

_expires_at: explicit Unix timestamp indicating expiration.

Key features include:

Expired documents are automatically excluded from query results, ensuring they appear as if removed.

A purge_expired() method that allows manual cleanup of expired documents from the storage.

Complete test coverage verifying correct filtering, purging, and TTL behavior.

Documentation additions in usage.rst outlining how to use the TTL middleware with code examples and notes on caching.

This feature addresses issue #599 to enable native document TTL support within TinyDB.

All tests pass locally with pytest including coverage measurements.