secrets with no secret store references should behave like plain strings

[redbaron]

Use Case

secretstore memguard is quite complicated, but it is invoked every time "secret-capable" config option is accessed even if there is no references to a secret store.

There is no value in "protecting" what essentially a plaintext string.

Expected behavior

config parameters with no secret store references should be short-circuit to a plaintext string, skipping memguard completely.

Actual behavior

memguard is used every time config parameter with potential secret in it is accessed, even if there is no references to secret stores.

Additional info

Memguard can be quite challenging to deal with: #13804, #13806. Skipping it entirely when it doesn't add any value simplifies telegraf setup.

[powersj]

next steps: similar to #13804, want to get on the same page with Sven and go through each of these.