Add support for routing items to a specific Monday group (group_id)

Issue summary:

Problem / Motivation

The Webform handler creates items via `create_item` but never passes
`group_id`, so every submission lands in the board's top (default) group.
On a board with multiple groups (e.g. `New Lead`,
imported CSV groups), there is no way to route submissions to a specific
group from configuration.

Proposed resolution

Add an optional **Group ID** textfield to the handler configuration form,
sitting under **Board ID**. When set, pass it as the `group_id` variable
of the `create_item` mutation. When empty, behaviour is unchanged (Monday
drops the item into the default group).

API change

`MondayClient::createItem()` gains an optional `?string $groupId = null`
parameter, threaded into the GraphQL mutation variables.

UI change

One new textfield on the handler config form:

- **Group ID** — optional. Numeric or string ID from the board's group
(e.g. `topics`, `group_mm4kpmas`). Leave empty to use the board default.

Remaining tasks

- Add `group_id` to `defaultConfiguration()`, `buildConfigurationForm()`,
`submitConfigurationForm()`, and the schema YAML.
- Thread it through `createItem()` and the GraphQL query string.
- Update README.

Want me to tweak the tone (less prescriptive in the "Proposed resolution" so reviewers can
push back), or shorten further?