allow setting the session pool size in niquests

[sjoeboo]

Description

adds ability to set the session pool size in niquests when needed:

grafana = GrafanaApi.from_url(
    url=GRAFANA_URL, credential=TokenAuth(token=GRAFANA_API_TOKEN)
)
grafana.client.timeout = 60
grafana.client.session_pool_size = 32

Checklist

• [ ] The patch has appropriate test coverage
• [ ] The patch follows the style guidelines of this project
• [ ] The patch has appropriate comments, particularly in hard-to-understand areas
• [ ] The documentation was updated corresponding to the patch
• [ ] I have performed a self-review of this patch

[sjoeboo]

addresses #189

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.29%. Comparing base (7303ec2) to head (dcca2ae). Report is 48 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #190 +/- ## ========================================== - Coverage 94.44% 92.29% -2.16% ========================================== Files 26 27 +1 Lines 1711 1816 +105 ========================================== + Hits 1616 1676 +60 - Misses 95 140 +45 ```

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