Add top-level GraphQL query for a single board list

What does this MR do?

Related to #340765 (closed)

This MR adds the ability to find a board list by gid. It is needed for getting some list information separately on the frontend: !62776 (closed).

Example query

query {
  boardList(id: "gid://gitlab/List/17", issueFilters: {authorUsername: "root"}) {
    title
    issuesCount
  }
}

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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