[contextual_tasks] Add service and component for contextual tasks.

This CL introduces a new component, `contextual_tasks`, which provides a
service for managing contextual tasks. A contextual task represents a
user's journey to accomplish a goal.

The `ContextualTasksService` is a `KeyedService` that allows clients to
create, retrieve, and delete tasks. Each task is identified by a unique
ID.

The service is created through `ContextualTasksServiceFactory` and is
associated with a user's profile. It is not available in incognito
mode.

This CL includes the public API, the internal implementation, the
factory, plumbing, and unit tests for the new component.

R=sophiechang@chromium.org

Bug: b:443652177
Change-Id: Ia4f993c839339367d61b2429816481cc5687ef3f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6921578
Commit-Queue: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1513079}