feat(query): added Query::readonly method

[vwency]

Summary

Closes #343

[abonander]

Strictly speaking, readonly is not a binary setting: https://clickhouse.com/docs/operations/settings/permissions-for-queries#readonly

This would be best represented by an enum like this:

pub enum ReadOnly {
    Disabled,
    Enabled,
    WithChangeSettings, // bikeshedding
}

You could make it possible to still pass a bool if you did something like this:

impl Query {
    pub fn readonly(mut self, readonly: impl Into<ReadOnly>) -> Self { ... }
}

impl From<bool> for ReadOnly {
    fn from(readonly: bool) -> Self {
        if readonly {
            Self::Enabled
        } else {
            Self::Disabled
        }
    }
}

[abonander]

Please do not modify existing CHANGELOG entries; this will just confuse people trying to understand older releases because this does not exist in 0.14.1. You can either add a new entry under ## Unreleased or we will write one when cutting the release.