Implement /api/external_engine endpoints

[Anupya]

Description

Implements the following:

• get = GET /api/external-engine
• get_by_id = /api/external-engines/{id}
• create = POST /api/external-engine
• update = PUT /api/external-engine
• delete = DEL /api/external-engine

Note that this PR does NOT include any of the endpoints that use a different domain than https://lichess.org.

After merge

Please check off the above endpoints in the issue description for #6 🙏

[benediktwerner]

Same as #50, if possible, please use typed dicts for return types.

On a side note, maybe also would be nice to mention whether and which authorization is required for an endpoint. It's usually mentioned in the Lichess API docs under "Authorization".

[Anupya]

Please review!

[kraktus]

Thanks! Nice work