feat(extensions)!: improve argument handling

[wackywendell]

Description

This changes the ExtensionArgs interface to follow standard Option usage / naming more closely, for ease of use:

fn from_args(args: &ExtensionArgs) -> Result<Self, ExtensionError> {
    let mut args = args.extractor();
    let path = args.expect_named::<&str>("path")?.to_owned();
    let batch_size = args.get_named::<i64>("batch_size")?.unwrap_or(1024);
    args.check_exhausted()?;
    Ok(Self { path, batch_size })
}

Also updates to error handling for extension errors, the addition of ExtensionValue::Null, and some GRAMMAR.md updates

Changes

• Replaces get_named_or with get_named::<T>() -> Result<Option<T>, ExtensionError> and renames expect_named_arg to expect_named.
• Includes the argument name in conversion errors, for example Invalid named argument 'count': ....
• Adds ExtensionValue::Null for a present, untyped null extension value. Typed nulls such as null:i64? remain expressions.
• Adds ExtensionValue::Error, allowing one malformed field to render as a local failure token without losing later arguments.
• Added Fetch and Cast to GRAMMAR.md, since they weren't there...

API impact

This is a breaking public API change:

• get_named_or is removed (in favor of get_named(...).unwrap_or(...), more standard Option naming) and expect_named_arg is renamed to expect_named.
• ExtensionValue and ExtensionValueKind have new Null and Error variants, which affects exhaustive matches.