feat: implement feeds endpoint

[aronza]

Summary:

Implements:

• Develop feeds endpoint #32
• Develop feeds/{id} endpoint #35
• Develop metadata endpoint #51
• fixes the broken tests by introducing environment variables during the tests and fix endpoints

Expected behavior:

Screenshots from Postman requests against local API:

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[jcpitre]

There are also some query parameters to implement, namely limit and offset.
filter and sort have been delegated to #63
I suggest either you mention clearly in this PR and in the issue that limit and offset are not included, and not close the issue after the PR is closed. or you create a new issue to cover these parameters.

[cka-y]

Nice to have: we should probably set up a mechanism to have a single instance of the database (singleton pattern). Instantiating the database as a global variable might increase the risk of concurrency issues and creates a lack of control over initialization in case we need to make any changes to it.

[aronza]

moved to a global variable in database module

[cka-y]

Suggestion: this function could be added to a utils file/module if you think it's going to be used by other classes or added as a static method of the FeedApiImpl class otherwise to keep the coding style consistent.

[aronza]

moved to a static method in this class as this looks specific to this module

[aronza]

There are also some query parameters to implement, namely limit and offset. filter and sort have been delegated to #63 I suggest either you mention clearly in this PR and in the issue that limit and offset are not included, and not close the issue after the PR is closed. or you create a new issue to cover these parameters.

Added limit and offset to this PR. filter and sort can be tracked separately as these apply to most endpoints

[aronza]

@cka-y we saw that you added changes to provider field in feat_15.sql. The OpenAPI spec still shows providers as a array of strings on BasicFeed. I've changed it to provider as a string

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