support legacy [plain_text_auth] section in credentials file removed unintentionally

The pylib/cqlshlib/cqlshmain.py code reads data from the credentials file, however, it is immediately ignored.

https://github.com/apache/cassandra/blob/c9625e0102dab66f41d3ef2338c54d499e73a8c5/pylib/cqlshlib/cqlshmain.py#L2070

    if not options.username:
        credentials = configparser.ConfigParser()
        if options.credentials is not None:
            credentials.read(options.credentials)        # use the username from credentials file but fallback to cqlshrc if username is absent from the command line parameters
        options.username = username_from_cqlshrc    if not options.password:
        rawcredentials = configparser.RawConfigParser()
        if options.credentials is not None:
            rawcredentials.read(options.credentials)        # handling password in the same way as username, priority cli > credentials > cqlshrc
        options.password = option_with_default(rawcredentials.get, 'plain_text_auth', 'password', password_from_cqlshrc)
        options.password = password_from_cqlshrc

These corrections have been made in accordance with https://issues.apache.org/jira/browse/CASSANDRA-16983 and https://issues.apache.org/jira/browse/CASSANDRA-16456.

The documentation does not indicate that AuthProviders can be used in the cqlshrc and credentials files.

I propose to return the ability to use the legacy option of specifying the user and password in the credentials file in the [plain_text_auth] section.

It is also required to describe the rules for using the credentials file in the documentation.

I can make a corresponding pull request.

EDIT by Stefan Miklosovic:

specifying username and password in credentials file works, it is just that [plain_text_auth] section does not work in credentials file anymore. This was working with CASSANDRA-16983 but it stopped to work by CASSANDRA-16456. Both tickets were firstly introduced in 4.1.0 (for the public). I do not think that it was ever an intention to stop to support that when CASSANDRA-16456 was merged and it was most probably just overlooked.