feat(monitoring): Use env variable as user agent

[salome-voltz]

Context

Add possibility to change user agent in requests.

PR check list

• [ ] As much as possible, the changes include tests (unit and/or functional)
• [ ] If the changes affect the end user (new feature, behavior change, bug fix) then the PR has a changelog entry (see doc/dev/getting-started.md). If the changes do not affect the end user, then the skip-changelog label has been added to the PR.

[codecov[bot]]

Codecov Report

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

Project coverage is 91.55%. Comparing base (5a57931) to head (921e54a). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #961 +/- ## ======================================= Coverage 91.54% 91.55% ======================================= Files 178 178 Lines 7372 7373 +1 ======================================= + Hits 6749 6750 +1 Misses 623 623 ``` | [Flag](https://app.codecov.io/gh/GitGuardian/ggshield/pull/961/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/GitGuardian/ggshield/pull/961/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian) | `91.55% <100.00%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian#carryforward-flags-in-the-pull-request-comment) to find out more.

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