Support Secret variables

[Lakitna]

Suggestion by @GrootKevin.

Robot supports the special variable type Secret. A Secret will never show up in the logs.

Support storing and retrieving Secrets values and Secret collection values.

Analysis of current state by Kevin (translated from Dutch):

• Storing Secret object is not possible
• Storing Secret.value does work since it's just a string (in my case)
• Retrieving cached value always returns a value of type string (or some other supported type) but in some cases you want it to return a Secrete object so it won't be logged.

[GrootKevin]

Hi @Lakitna, Thanks for opening an issue for this specific request!