Add hash field expiration support for Redis 7.4+

[abrookins]

Summary

Adds support for Redis 7.4+ hash field expiration to HashModel.

Closes #750
Fixes #753

Changes

New Features

• Field(expire=N) parameter to set default TTL (in seconds) for hash fields
• model.expire_field(field_name, seconds) - Set TTL on a specific field
• model.field_ttl(field_name) - Get remaining TTL of a field
• model.persist_field(field_name) - Remove expiration from a field
• model.save(field_expirations={...}) - Override default expirations at save time
• supports_hash_field_expiration() function for version checking

Bug Fix (#753)

• save() now preserves manually-set field TTLs
• HSET removes field-level TTLs, so we now get TTLs before HSET and restore them after

Requirements

• Redis 7.4+ for hash field expiration commands (HEXPIRE, HTTL, HPERSIST)
• redis-py 5.1.0+ for Python client support

Example Usage

from aredis_om import HashModel, Field

class Session(HashModel, index=True):
    user_id: str
    token: str = Field(expire=3600)  # Expires in 1 hour

session = Session(user_id="123", token="abc")
await session.save()

# Check TTL
ttl = await session.field_ttl("token")  # Returns ~3600

# Manually extend TTL
await session.expire_field("token", 7200)

# Remove expiration
await session.persist_field("token")