Add support for capturing warnings and logging them via Streamlink's
root logger on the warning log level. Use custom `WarningLogRecord`s
with a custom warning message format, and replace the record's
logger name with "warnings" and use the name of the warning type
as log level name when showing warnings.
This enables having proper `DeprecationWarning`s and `FutureWarning`s
in the code instead of just using the logger and its warning log level,
and those warnings can be filtered via the regular filtering mechanisms.
For example:
```
[warnings][deprecationwarning] Calling this method is deprecated
[warnings][futurewarning] Using this config file path is deprecated
```

- Replace all deprecation warning log messages with `FutureWarning`s
- Fix deprecated session options in tests
- Update and fix tests