[FR] Allow poolsize and maxpool size settings for http client

Issue に示されている HttpClient コンストラクターから始め、どのようにその requests.Session が両方のスキームに対して HTTPAdapter インスタンスを mount しているかを調べます。既存の retry、timeout、session の設定を追跡し、構成可能な pool 設定が HTTP と HTTPS のリクエストに一貫して適用されること、またデフォルトではクライアントの動作が変わらないことを確認します。

Is your feature request related to a problem? Please describe.
I am currently facing a performance issue in which the default max pool size of requests library has become a bottleneck.

Describe the solution you'd like
A clear and concise description of what you want to happen.
I was wondering if we could increase the pool size by taking them as parameters for message client constructor
Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context

class HttpClient:
    def __init__(
            self, credential=None, session=None, base_url='', headers=None,
            retries=DEFAULT_RETRY_CONFIG, timeout=DEFAULT_TIMEOUT_SECONDS,
            pool_connections=DEFAULT_POOL_CONNECTIONS, pool_maxsize=DEFAULT_MAX_POOL_SIZE
        ):
        if credential:
            self._session = transport.requests.AuthorizedSession(credential)
        elif session:
            self._session = session
        else:
            self._session = requests.Session() # pylint: disable=redefined-variable-type

        if headers:
            self._session.headers.update(headers)
        if retries:
            self._session.mount('http://', requests.adapters.HTTPAdapter(max_retries=retries))
            self._session.mount('https://', requests.adapters.HTTPAdapter(max_retries=retries))
        
        self._session.mount('http://', requests.adapters.HTTPAdapter(pool_connections=pool_connections, pool_maxsize=pool_maxsize))
         self._session.mount('https://', requests.adapters.HTTPAdapter(pool_connections=pool_connections, pool_maxsize=pool_maxsize))
        
        self._base_url = base_url
        self._timeout = timeout