User access token creation date

## What?

User access tokens should record and display the date and time they were created.

## Why?

Knowing when a specific access token was created can help users and administrators better identify the access token (perhaps the user wasn't specific with their description), identify access tokens suitable for rotation, etc.

Currently, only `last_access` is tracked (https://github.com/Graylog2/graylog2-server/blob/de6421786490acbe091ec80726c18a986230adc2/graylog2-server/src/main/java/org/graylog2/security/realm/AccessTokenAuthenticator.java).

Related to #21198

## Your Environment

* Graylog Version: `6.1.4`