Edit Custom Fields without enrollment or renewal

[cdmadrigal]

BUSINESS PROBLEM
There are times that custom field values on a certificate need to be edited or added before the renewal window comes. For example, a custom field for the certificate owner may need to change due to changes within the team. We would not want to have to renew the certificate to make these changes.

PROPOSED SOLUTION
Support a vcert command to write/edit custom fields within a certificate object.
Example:
vcert customfield -t <token> --id <id> --field <field>

CURRENT ALTERNATIVES
Use the API directly.