Allow a custom HTTP client to be used with ServiceReservation

[cjh1]

Checklist

• My code follows the style guidelines of this project
• I have added/updated comments where needed
• I have added tests that prove my fix is effective or my feature works
• I have run make test (or equivalent) locally and all tests pass
• DCO Sign-off: All commits are signed off (git commit -s) with my real name and email
• REUSE Compliance:
• Each new/modified source file has SPDX copyright and license headers
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• All referenced licenses are present in the LICENSES/ directory

Description

PCS uses the ServiceReservation interface to make reservations, we need to be able to initialize it with custom HTTP client which has an OAuth2 transport enabled.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

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For more info, see Contributing Guidelines.

[davidallendj]

LGTM