Add Referer Header when fecthing image medium

[JingMatrix]

Describe the bug

Some images hosting services such as https://b-1254719278.cos.accelerate.myqcloud.com/wp-content/uploads will refuse to load images if no valid Referer header is present in the request.

To Reproduce
An examplar feed is: https://bedtimepoem.com/feed/atom
To see the issue, the command

curl -I 'https://b-1254719278.cos.accelerate.myqcloud.com/wp-content/uploads/2023/07/WechatIMG19-1024x1024.jpg'

returns 403.
But the command

curl -I 'https://b-1254719278.cos.accelerate.myqcloud.com/wp-content/uploads/2023/07/WechatIMG19-1024x1024.jpg' \
  -H 'Referer: https://bedtimepoem.com/archives/17372'

returns 200 correctly.

Expected behavior
rsstt should add the Referer header so that images can be fetched reliably.

Important log

Traceback (most recent call last):
  File "/home/jing/.local/python/lib/python3.11/site-packages/rsstt/web.py", line 457, in get_medium_info
    raise ValueError(f'status code is not 200, but {r.status}')
ValueError: status code is not 200, but 403
2023-07-21-16:21:12:DEBUG:RSStT.web - Medium fetch failed: https://b-1254719278.cos.accelerate.myqcloud.com/wp-content/uploads/2023/07/WechatIMG19-1024x1024.jpg
Traceback (most recent call last):
  File "/home/jing/.local/python/lib/python3.11/site-packages/asyncstdlib/_lrucache.py", line 407, in __call__
    result = self.__cache[key]
             ~~~~~~~~~~~~^^^^^
KeyError: 'https://b-1254719278.cos.accelerate.myqcloud.com/wp-content/uploads/2023/07/WechatIMG19-1024x1024.jpg'

[Rongronggg9]

Sounds reasonable. However, I am pretty occupied these days so PR is welcomed.

[JingMatrix]

Hi! Do you have related development documents? I didn't find them yet.

Most importantly, how do you set up a test environment locally?

[Rongronggg9]

Hmm, it seems that only RSStT fetching images with proper referer is not sufficient. RSStT relies on Telegram DC to finally fetch and send images. That RSStT itself succeeds in fetching images does not mean the success of Telegram DC, while it is the latter that is the key to sending images.
To fix the issue, https://github.com/Rongronggg9/rsstt-img-relay needs to accept custom HTTP headers first. Afterward, RSStT can benefit from it.