Unify --allow-unsupported-set and --unsupported-set-mode

[altmannmarcelo]

Description

Today we have --unsupported-set-mode that allows for two values, Error and Proxy. but we also have --allow-unsupported-set as a dedicated variable. Those two variables all control what we set in:

pub enum UnsupportedSetMode {
    /// Return an error to the client (the default)
    Error,
    /// Proxy all subsequent statements to the upstream
    Proxy,
    /// Allow all unsupported set statements
    Allow,
}

We should unify them by adding allow as one option to --unsupported-set-mode

Change in user-visible behavior

Requires documentation change