feat: added /stakeholder/stakeholder_id endpoint

[XLPeng57]

Added endpoint for #372

fixed: #372

[dadiorchen]

When you use: fixed: #xxx it will link the PR with issue, and automatically close issue when pr get merged

[dadiorchen]

The unit test looks great.

We didn't set up a good base for this repo, so we just do unit test to cover the repository class, ideally we should use integration test to cover code crossing files.

So did you try to run the server and tested the endpoint?

[dadiorchen]

Great job, thank you for your contribution!