feat: download OneAgent installers

[Flarna]

What would you like?

I would like to use a command line tool to download, verify signature and unpack OneAgent installers (full stack, paas,...).

Likely an API to fetch available versions would be also helpful.

See
https://docs.dynatrace.com/docs/dynatrace-api/environment-api/deployment/oneagent/download-oneagent-latest
https://docs.dynatrace.com/docs/dynatrace-api/environment-api/deployment/oneagent/download-oneagent-version
https://docs.dynatrace.com/docs/dynatrace-api/environment-api/deployment/oneagent/get-available-versions

Example usage

dtctl get oneagent --installer-type paas
dtctl get oneagent --flavor musl
dtctl get oneagent --installer-type paas --os-type windows --version 1.155.275.20181112-084458

[discostu105]

The relevant REST APIs for this are currently only available via APIs that are on classic API tokens. They will be enabled to get Platform Token support. As soon as this is available, it would be sense to add that.