Add an MGI allele endpoint

[dyf]

• [ ] figure out how to query MGI's allele database by allele name
• [ ] set up http://aind-metadata-service/mgi_allele/[allele_name] endpoint
• [ ] given an allele name, return a aind-data-schema Allele record

[jtyoung84]

@dyf Can you add some additional acceptance criteria?