feat: add support for https_proxy environment variable (#415)

[recreators01]

https_proxy should be enough.
Regarding the changes to umu_runtime.py, the URL must include a scheme when using ProxyManager; otherwise, it will not work.

[loathingKernel]

Please rebase your PR properly on top of current main branch to avoid merge artifacts. Also avoid unnecessary style changes in unrelated code.

[recreators01]

Please rebase your PR properly on top of current main branch to avoid merge artifacts. Also avoid unnecessary style changes in unrelated code.

ok

[recreators01]

Close:#415
This also resolves the issue that URLs without a scheme (i.e., 'https://') are deprecated and will raise an error in urllib3 v3.0.

[loathingKernel]

Is lowercase https_proxy a common env variable or something you came up with for this PR? If it isn't common, IMO it would better to be uppercase, and specific to UMU, i.e. UMU_HTTP_PROXY to match the existing UMU_HTTP_RETRIES and UMU_HTTP_TIMEOUT

[recreators01]

Common env variable. Most app use it, e.g. curl wget git python urllib/urllib2.

[loathingKernel]

I see, for future reference this is the rationale curl provides: https://everything.curl.dev/usingcurl/proxies/env.html#http_proxy-in-lower-case-only