Make it easier to modify metas for generic PartiqlAst.Expr

[alancai98]

When trying to transform PartiqlAst.Expr's metas, there currently isn't an easy way to change or add metas (for the generic PartiqlAst.Expr).

A current workaround is to create an inner class that overrides the PIG transformMetas function. i.e.

inner class MetaVisitorTransform(private val newMetas: MetaContainer) : PartiqlAst.VisitorTransform() {
    override fun transformMetas(metas: MetaContainer): MetaContainer = newMetas
}

An alternate workaround to add/edit metas can be to case on the specific PartiqlAst.Expr type and call its corresponding withMeta function.

It would be nice to have a .copy function that ExprNode currently has. Alternatively, there could be a withMeta function that PartiqlAst.Expr can call.

A more ideal solution would likely involve editing the PIG PartiqlAst type domain for PartiqlAst.Expr.