Client certificate dialog tweaks

Our existing client certificate dialog in datanode is rather clumsy and confusing. There are several things we could implement to make it really user friendly.

## What?
* [x] [Unencrypted certificate handling](https://github.com/Graylog2/graylog2-server/pull/22550)
* [ ] [Selector of available opensearch roles](https://github.com/Graylog2/graylog2-server/pull/22550)
* [ ] [Support for selecting multiple roles](https://github.com/Graylog2/graylog2-server/pull/23370)
* [ ] [Show list all existing principals and their roles](https://github.com/Graylog2/graylog2-server/pull/22550)
* [ ] Button for deleting a principals and their role mappings
* [ ] Download a bundle of certificates and keys, instead of copy-paste fields

## Why?
Current dialog is very basic and users are struggling to use it properly.