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

**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
```