Allow custom field dropdowns to use more than 100 options

Issue Summary

Custom fields with more than 100 values will only allow you to use the first 100 values in the /jira create dialogue. This is due to Slack's static multi-select option limit of 100.

Suggestion

If a custom field in Jira has more than 100 values, use the external data source input type to allow dynamic lookups in the field.