text: Title: Support controlling separator when appending current value using `Set Suite Metadata`, `Set Test Documentation` and other such keywords
  Body: Two deficiencies identified with the current Set Suite Metadata keyword, which we use to record stuff like version numbers.
  
  - [x] Add argument to define a separator between multiple metadata returned values when append=True. Currently, the default behavior is just to put in a space. 
  
  https://robotframework.org/robotframework/latest/libraries/BuiltIn.html#Set%20Suite%20Metadata
  > Arguments:
  > name
  > value
  > append = False
  > top = False
  > 
  
  ### Example
  **current:** 
  ```
      FOR    ${device}    IN   @{devices}
          Set Suite Metadata | Versions | ${version} | append=True
      END
  ```
  **output** 
  Versions: 1.0 1.2 1.0 1.0 2.3
  
  **proposal:**
  ```
      FOR    ${device}    IN   @{devices}
          Set Suite Metadata | Versions | ${version} | append=True  | separator=,
      END
  ```
  **output** 
  Versions: 1.0, 1.2, 1.0, 1.0, 2.3
  
  - [ ]  Argument to allow unique returned values only
  To continue the example above:
  ```
      FOR    ${device}    IN   @{devices}
          Set Suite Metadata | Versions | ${version} | append=True  | separator=, | unique=True
      END
  ```
  **output** 
  Versions: 1.0, 1.2, 2.3