Ability to Cache the public key of another atSign

[XavierChanth]

Is your feature request related to a problem? Please describe.

We had to manually implement a version of this feature into sshnp client.

Describe the solution you'd like

@gkc has enabled caching of the sshnpd atSign's public key but without having (built in AtClient) local store enabled, it would be useful to have the ability to signal to the client to cache the public key of another atSign or a few atSigns without the need to create a full local store. This is especially useful for applications like sshnp, which are primarily driven by real-time events and want to use remote notifications instead of using persistent storage with syncing and a cache.

Describe alternatives you've considered

No response

Additional context

No response

[XavierChanth]

P3 for now - we have a working implementation in sshnp client for now, we can abstract it away once we have another request for it in another project

Also, it may fall under this: #606