Shallow merge meta with defaults

[backbone87]

When having default options with meta, the meta object is completely replaced when "later" options also have a meta key. This can be quite cumbersome when using meta as a "DI container" , e.g.:

// client.ts
export const FETCH = Symbol('FETCH');

declare module '@tanstack/react-query' {
  interface Register {
    queryMeta: {
      [FETCH]?: typeof globalThis.fetch;
    };
    mutationMeta: {
      [FETCH]?: typeof globalThis.fetch
    };
  }
}

const fetchWithTracing = createFetchWithTracing(globalThis.fetch);
const client = new QueryClient({
  defaultOptions: {
    queries: {
      meta: {
        [FETCH]?: fetchWithTracing,
      }
    },
    mutations: {
      meta: {
        [FETCH]?: fetchWithTracing,
      }
    },
  },
});

// SomeComponent.ts
const fetchWithTracing = useContext(FetchWithTracingContext);
const {} = useQuery({
  meta: {
    [FETCH]: fetchWithTracing, // !!! have to respecify the fetch client, because we also want to pass other meta props
    querySpecific: 'Foo',
  }
})

I think it would be better if meta objects are automatically merged with default/common options. This might be a breaking change so i see 3 options how to do that:

1. BC break is reasonably edge case to just introduce the merging behavior
2. Be able to pass a function to meta which receives the "default meta", e.g. { meta: (meta: QueryMeta | undefined) => ({ ...meta, querySpecific: 'Foo' }) }
3. Introduce container option that has merging behavior by default and otherwise behaves like meta (maybe deprecate meta)