Add ability to set timeout in Session objects

[syeopite]

• Explain the use-case carefully. Maybe with a code example
  It makes sense to be able to set a universal timeout that all requests made with the session can adhere to. This avoids having to pass a custom timeout for every single request

async with AsyncSession(timeout=50) as s:
    # Has a timeout of 50 seconds
    resp = await s.get('/v1/users')

• Who needs this?
• What pain does this resolve?
  This prevents having to pass the timeout parameter on every single request
• Is this standard?
  Yes. Both httpx and aiohttp for example allows setting a timeout on their respective client objects

[Ousret]

Reasonable. It will be available in 3.14.0