merge: Support sequences without cross-checking

[victorlin]

Description of proposed changes

Initial prototype for sequence support in augur merge where metadata and sequence merge happens without any cross-checking.

Related issue(s)

Closes #1579

Checklist

• Address FIXMEs
• Add seqkit as an external dependency (c099e16)
• Automated checks pass
• Check if you need to add a changelog message
• Check if you need to add tests
• Check if you need to update docs

[victorlin]

Closing – the latest plan is to support at least some form of optional cross-checking.