Add custom endpoint for GET posts

[sidemt]

Checklist:

• I have read freeCodeCamp's contribution guidelines.
• My pull request has a descriptive title (not a vague title like Update index.md)

Partially implements #94

This PR adds the following 3 endpoints replicating the current API:

• GET /content/posts/
• GET /content/posts/{id}/
• GET /content/posts/slug/{slug}/

Each endpoint accepts query parameter include=authors,tags to include author and tags in the response.

[raisedadead]

@yoko consider adding unit tests. That way we can mock the shape of the response and make sure these endpoints respect the shape that is expected (that is content is similar to the current content API)

[socket-security]

New dependencies detected. Learn more about Socket for GitHub ↗︎

Packages	Version	New capabilities	Transitives	Size	Publisher
sqlite3	5.1.6	network, shell	+56	8.14 MB	daniellockyer
supertest	6.3.3	network	+8	788 kB	titanism

[sidemt]

@raisedadead I tried adding tests according this document: https://docs.strapi.io/dev-docs/testing#set-up-a-testing-environment

However, the test environment uses sqlite3 for database and I've been unable to find the way to use postgres instead. Also, I can't find how I can integrate this test setup with our Docker setup. (I'm not even sure if we need to do so.)

Could you take a look at the current code and see if I'm going the right path?

With the current code, I can run the test from the console like this:

[ojeytonwilliams]

Hey Yoko, I just noticed a few code style things that could be improved:

[raisedadead]

...the test environment uses sqlite3 for database and I've been unable to find the way to use postgres instead.

That is perfectly fine, as none of our code is Postres-specific. We are good here because we will do everything using Strapi's backend.

[sidemt]

Thank you for your review, @ojeytonwilliams @raisedadead

I have applied Oliver's suggestions and added one more test case to check the behavior of the include query parameter.

[sidemt]

Getting this change in for now. If there are any issues, I'll work on them in a separate PR. Thanks.