Add fortigate_exporter to community

[Philldomd]

We would like to move https://github.com/bluecmd/fortigate_exporter here

It was previously maintained by bluecmd, but the project is archived.

We would like to continue development on this exporter but would like it to live on here.

We have been in contact with @SuperQ, who suggested to lift it in under this community.

[SuperQ]

Before this exporter is moved, it needs to be re-licensed Apache-2.0.

[SuperQ]

The relicense has been completed. I've created a fortigate_exporter GitHub team for managing permissions.

[SuperQ]

Also, for the record, I am able to sponsor this as a Prometheus Team member.