ovsx-client: Use get for querying extension data

[msujew]

What it does

In order to improve cache-ability, a few weeks back osvx implemented the option to query extensions by using get (see eclipse/openvsx#331).

This change aligns Theia to this decision by using get instead of post requests. Additionally allows to generically stitch together url-params. (compared to the previous manual approach)

How to test

1. Open the Extensions view
2. When searching extensions, Theia should use get requests (confirm this using your network resources tab in your browsers dev-tools)
3. Other features of the ovsx client should behave as normal

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

[vince-fugnitto]

The changes look good to me and work well 👍

Should we remove the following as it is currently unused?

const postJson = bent('POST', {
    'Content-Type': 'application/json',
    'Accept': 'application/json'
}, 'json', 200);

protected postJson<P, R>(url: string, payload: P): Promise<R> {
    return postJson(url, JSON.stringify(payload)) as Promise<R>;
}

[msujew]

Should we remove the following as it is currently unused?

Sure, didn't notice they were unused 👍

Added an appropriate breaking change entry in the changelog.

[vince-fugnitto]

Looks good to me 👍
The next step would be to replace bent with a library that respects requests behind a proxy (#10267).