having field aliases - data available in multiple fields but only saved once

## What?
Be able to have multiple names for the same field.

## Why?
In multi-tenant environments it might be that each has its own normalization schema for data. All are working with the same data but the access should be via different names.

This would also allow organisations to make changes to the schema and transit over to newer definitions without a hard cut.

With that kind of ability in Graylog users could stick to the Graylog Schema but are not forced to align all internal tools and processing to that.

[z#844297]