Add a new type secret in Survey Variable

[sparsick]

Sometimes you want to add a password etc. and hide their value. Therefore, I add a new type 'secret' so this input field will hide the typed value is hidden.

Relates to #904

[sparsick]

@fiftin Is this PR okay, or should I do more so that it can be merged?

[joseangel-piqueras]

I need this feature! 💯

[tothakos-code]

This feature could be misleading. It just hides the input and doesn't handling the actual value as a secret thus someone password will be saved in clear text. The value is easily readable inside the browsers network tab.

My improvement idea:

• The secret survey "key:value" pairs would be sent to the backend in a different key than "environment" thus it will be separated.
• Handling the secrets on the backend only need to differ from environments in one place: not saving them to the db.
• This will break the rerun function, because there will be missing variables, so secret survey fields should always be empty and required for a rerun.

I didn't look into how hard it is to implement this idea or if it's even possible. I might later this week, when i have time.

[fiftin]

Merged @tothakos-code solution. Thank you.