Fix proxy environment variables not used in v1.0

[Wauplin]

Solve huggingface/transformers#41301 reported by @jerryzh168.

The root cause of this issue was when using a custom transport, the proxy env variables are not used by httpx even if trust_env=True. This is the case since encode/httpx#1122. We were using custom Transport in order to handle HF_HUB_OFFLINE env variable (i.e. raise exception if attempt to make an HTTP call) and add custom request id header to help with debugging.

The workaround in this PR is to use event_hooks instead. We are still catching offline mode + adding debug logs at runtime but don't use custom transport anymore. I've added a regression test to check that HTTP_PROXY/HTTPS_PROXY are correctly read from env.

Note: there is a drawback to this PR. We do not catch any httpx.RequestErrors anymore to add the request id to the error message itself. This is not possible to do without a custom Transport. In practice it's not much of an issue because the request id is added back by hf_raise_for_status. The only regression is if an error happens before the server respond (i.e. a readtimeout or connection error), in which case we won't have a request id in the error message. The benefits of this PR still outweigh this drawback IMO.

[HuggingFaceDocBuilderDev]

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[hanouticelina]

the fix looks good to me! 👍 okay to merge once the test is fixed