Read config files as UTF-8

[RizgarOzan]

set_conf_files reads .ini files with ConfigParser.read() and .json files with a bare open(), so both use the locale encoding. On Windows that's cp1252/cp1254 and so on, which turns a UTF-8 config value into mojibake:

# ~/.config/fsspec/conf.json, saved as UTF-8: {"proto": {"cache": "C:/Users/Müller/cache"}}
>>> fsspec.config.conf["proto"]["cache"]
'C:/Users/Müller/cache'

Both now read the files as UTF-8 (JSON has to be UTF-8 anyway), and the config docs say so. The new test fails on master on Windows; on Linux CI it passes either way since the locale there is already UTF-8.

Tested on Windows (Python 3.12, cp1254 locale): pytest fsspec/tests/test_config.py goes from 1 failed to 8 passed, and the full pytest fsspec run fails on the same tests as master. pre-commit is clean.