Add timestamp to jsonschema

[nvdaes]

Summary of the issue

Add-on metadata may accept a timestamp, to sort add-ons or perform other queries.

Development approach

• Update the json schema to accept an optional "submissionTime", corresponding to a timestamp. We use milliseconds instead of seconds, in case similar values are added in the future, for a better comparison.
• When creating a json file, the current time will be assigned to submissionTime.
• Updated tests, removing the submissionTime before comparing actual and expected json files.

[nvdaes]

We can add the timestamp using Python, or with GitHub Actions. I'll try to use Python from this repo.

[nvdaes]

@seanbudd , we now use mktime to build a timestamp from gmtime.

[seanbudd]

Thanks @nvdaes - we're going to hold off on merging until the other PRs are ready

[nvdaes]

I wil try to create a PR in NVda repo adding a combo box to the store settings to sort addons by date or alphabetically . This should be done, imo, in config.conf since it is related to all addons, in contrast with options to filter addons by fields in the store gui. I will start this weekend .