feat: group expressions by selectors

[spherel]

Description

This PR extends the selector scope for expressions. Previously, we are only allowed to set selectors for all read/write/lookup/zero expressions. In this PR, we change the definition of ConstraintSystem as

pub struct ConstraintSystem<E: ExtensionField> {
    ...
    pub expression_groups: BTreeMap<Option<SelectorType<E>>, ExpressionGroup<E>>,
    ...
    pub default_read_selector: Option<SelectorType<E>>,
    pub default_write_selector: Option<SelectorType<E>>,
    pub default_lookup_selector: Option<SelectorType<E>>,
    pub default_zero_selector: Option<SelectorType<E>>,
    ...
}

so that read (or write, lookup, zero) expressions can have different selectors.