Better compatibility with oidc-agent

[tdviet]

Initial checklist

• [X] I read the support docs
• [X] I read the contributing guide
• [X] I agree to follow the code of conduct
• [X] I searched issues and couldn’t find anything (or linked relevant results below)

Problem

Fedcloud client should try to read access token from OIDC_AT environment variable (default for oidc-agent) if FEDCLOUD_ACCESS_TOKEN or OIDC_ACCESS_TOKEN is not set. See discussion at https://github.com/tdviet/fedcloudclient/discussions/205

Solution

Add compatibility mode

Alternatives

No