Add possibility to filter extensions by API version

[gornostal]

Blocked by #1

We want to add possibility for users to filter extensions by Ulauncher Extension API version.

Let's say I'm using new Ulauncher v5 and I want to see what extensions already support it.
I want to go to ext.ulauncher.io and filter by API v2.0.0

On the API side we want to extend GET /extensions endpoint with an optional query param version

Technical details

• Add python-semver as a dependency
• If version query param is present, call satisfies(version, required_api_version) for each item in SupportedVersions list
• Add it to the returned list if there's at least one match
• If version param is empty, return all extensions