Allow STT checks file download manually

Currently, STT check work as follows,

Security checks developed independently from PMM releases, therefore PMM server downloads, via HTTPS, a list of available security checks from Percona servers each time before starting the STT cycle. Downloaded data is a single, signed YAML file.

Ref: https://www.percona.com/blog/2020/06/29/security-threat-tool-design-in-percona-monitoring-and-management/

FR:  Allow STT checks file download manually

 It will very useful for many users if we can download file checks manually, especially when running PMM-Server in environments with no external internet access.

User Usecase:

Would it be possible to manually download the checks by any means? And to put it in correct location on the PMM server.