Add type overloads for chat() method in _client.py

[simonottenhauskenbun]

This PR adds two type overloads in _client.py to reduce type warnings.
The PR should only affect typing and no logic.

Before PR:

import ollama
response = ollama.chat(model='llama2', messages=[
  {
    'role': 'user',
    'content': 'Why is the sky blue?',
  },
])
# before PR: response is now Union[Mapping[str, Any], Iterator[Mapping[str, Any]]]
print(response['message']['content'])

New behaviour:

response = ollama.chat(model='llama2', stream=False, ...)
# response is of type Mapping[str, Any]

response = ollama.chat(model='llama2', stream=True, ...)
# response is of type Iterator[Mapping[str, Any]]

response = ollama.chat(model='llama2', ...)
# response is also of type Mapping[str, Any], since stream=False is the default.