Use client side rate limiting for plugin download

[msujew]

What it does

Related to #11860. The publicly managed open-vsx instance is using a rate limiting of 15 requests per second (yielding a 429 status code on too many requests). This change introduces a rate limiter that limits the amount of requests that we send per second to 15, while also handling the 429 status code a bit better.

It also reinstates the parallel-by-default behavior of the plugin download command, which was changed in #11860. Additionally provides a CLI argument to change the rate limit.

How to test

0. The CI should not fail due to 429 errors even though parallel plugin downloads are enabled
1. Delete your plugins folder.
2. Run yarn download:plugins --rate-limit=x with different values for x
3. Assert that the rate limit is applied and the command doesn't return with the 429 http error

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[paul-marechal]

I tried with 15 and 1 as a rate limit and it worked.

Thanks for making it fast again!