Add support for canned responses to VS/VSR

[Rulox]

Proposed changes

This PR adds the "canned responses" features.

Now, it is possible to return preconfigured responses for any VS or VSR with the action return.

Also refactored the redirect action to reuse the same code as the new return action, as both make usage of the "return" directive from NGINX.

[pleshakov]

@Rulox thanks for the PR. good job!
I left a few suggestions regarding consistency and simplification. Let me know your thoughts.

[Rulox]

@pleshakov

Changed the validateSpecialVar to a switch case, with the next considerations:

isValidSpecialVariableHeader is needed but there's a difference between using - and _. Creating a new regexp for this is against DRY and KISS, reusing the header with replacing the underscores seems a much better and elegant solution to me.

I didn't create isValidSpecialVariableCookie or isValidSpecialArgument because those would have been just wrappers to isCookieName and isArgumentName so I think that would be just a waste of time and add code for nothing.

Let me know what you think thanks

[pleshakov]

@Rulox thank for the update.

Please see a few comments below. Additionally, there is a bug. sorry, only noticed that during this review.

Changed the validateSpecialVar to a switch case, with the next considerations:
I didn't create isValidSpecialVariableCookie or isValidSpecialArgument because those would have been just wrappers to isCookieName and isArgumentName so I think that would be just a waste of time and add code for nothing.

the switch looks good along with using the existing functions.

isValidSpecialVariableHeader is needed but there's a difference between using - and _. Creating a new regexp for this is against DRY and KISS, reusing the header with replacing the underscores seems a much better and elegant solution to me.

creating a new function is against DRY but not KISS. I left a comment for the corresponding line explaining why.

[pleshakov]

@Rulox please my comments and suggestions

[pleshakov]

👍