feat: Edit the requests configuration pool_maxsize variable when initializing client

- Algolia Client Version: 2.4.0
- Language Version: Python 3.9.1

### Description
I'm replicating a dataset (60k records) with 100 threads (Python's builtin _ThreadExecutor_) at a time. And I find that from time to time I keep getting these warnings from the 'requests' library: **"Connection pool is full, discarding connection: xxxxxxxx.algolia.net"**.

The ability to edit the _pool_maxsize_ configuration variable when initializing the search client would solve this kind of issues, perhaps even allowing to edit other _requests_ configurations.

### Steps To Reproduce
Initialize a ThreadExecutor of 100 max workers.
Create a function that uses 'save_object' to store a record in a predefined index.
Loop through thousands of records, for each record:
-Add the function that saves the record with the record data to the executor

You should get the warning every ~1000 records.

Start at the search client initialization path and trace how its requests session or connection pool is configured. Verify how concurrent save_object calls use that configuration; done means callers can set pool_maxsize when creating the client and the 100-worker reproduction no longer emits the pool-full warning.

Written by the indexing model from the issue text.