Add schema for unaligned sequences

[victorlin]

Description of proposed changes

Some workflows use unaligned sequences as input to augur subsample. In this case, proximal samples will not work properly. This schema makes it easier for workflows to guard against configs with proximal samples.

At run time, augur subsample itself still validates configs against the default schema that allows proximal samples. This is fine because augur proximity errors if sequence lengths differ.

Related issue(s)

• Use augur subsample rsv#103 (comment)
• Alternative to Add schema def for unaligned sequences #2037

Checklist

• Manually tested in Use augur subsample rsv#103
• Automated checks pass GitHub issues
• Check if you need to add a changelog message
• Check if you need to add tests
• Check if you need to update docs
• Add route for augur subsample config schema for unaligned sequences nextstrain.org#1368

[joverlee521]

Unfortunate it's not easy to layer JSON schemas for this, but I think having separate schemas here is worth it for workflows!