ENH: allow export to validate schema for a specific version of auspice

[corneliusroemer]

Context

I spent some time figuring out why tip_label default didn't work with Auspice.

Turns out that that feature was prematurely released in the auspice schema shipped with augur (#1299), before auspice supports that customization (see nextstrain/auspice#1692).

Description

While it's hopefully a one-off that augur's schema is ahead of auspice, it might be a good idea to allow validation against a range of auspice schema versions.

Use cases:

• When you want the auspice.json to be compatible with old auspice versions, one could e.g. --schema-versions 2.38,2.47 to export telling it to check that the auspice.json is valid for both auspice versions 2.38 and 2.47. By default we could use always the latest version at the time of the augur release.

I'm not sure where we currently get schemas from, it might be cool if schemas could be downloaded dynamically, so that new auspice schemas can be validated by old auspice versions.

We might even be able to point to a schema on a PR, if one wants to test a new feature. Maybe this could help prevent the premature addition of a future auspice capability in augur.