Improve builtin plugin sandbox

[lovromazgon]

Description

In this PR we change the behavior of the builtin plugin sandbox. Plugin functions are now run in a goroutine so we can detach from the call in case it blocks forever (e.g. a plugin bug). We add tests to confirm that this behavior matches the behavior in standalone plugins.

Quick checks:

• [X] I have followed the Code Guidelines.
• [X] There is no other pull request for the same update/change.
• [X] I have written unit tests.
• [X] I have made sure that the PR is of reasonable size and can be easily reviewed.