Add Timer::stop() API to make it consistent with Task.

[finger563]

Description

feat(timer): add stop() api to make it consistent with task - just an alias for cancel. Updated internal timer impl to use running_ variable for starting/stopping since the cv.wait_for/until was occasionally returning the wrong value when running in low power modes on the esp.

Motivation and Context

• Consistent APIs across similar components streamlines developer experience and debugging.
• Using the running_ variable ensures that even when running with low power modes (with light sleep enabled), we don't get issues with the cv.wait_for / wait_until returning the wrong status and causing the timer to exit.

How has this been tested?

Building and running the timer example.

Screenshots (if appropriate, e.g. schematic, board, console logs, lab pictures):

Correct behavior after the change:

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update
• Hardware (schematic, board, system design) change
• Software change

Checklist:

• My change requires a change to the documentation.
• I have added / updated the documentation related to this change via either README or WIKI

Software

• I have added tests to cover my changes.
• I have updated the .github/workflows/build.yml file to add my new test to the automated cloud build github action.
• All new and existing tests passed.
• My code follows the code style of this project.