Add commands to work with api testing project vaults

[mhan83]

Proposed changes

Adds the following commands to work with an api testing project vault:

saucectl apit vault get [--project PROJECT_NAME]
saucectl apit vault get-variable NAME [--project PROJECT_NAME]
saucectl apit vault set-variable NAME VALUE [--project PROJECT_NAME]
saucectl apit vault get-snippet NAME [--project PROJECT_NAME]
saucectl apit vault set-snippet NAME FILE_PATH|- [--project PROJECT_NAME]

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• I have updated the json schema (if appropriate)

Further comments

• Opted to make project a flag and not a positional arg. I opted for that because a command like get-variable feels like it wants the variable-in-question as the positional arg. Having the project be positional felt a bit awkward and unintuitive. But we already have some commands that have a "resource" as a positional arg, e.g artifacts download JOB_ID FILE_PATTERN. So if we prefer consistency, I'm happy to go that route.
• If --project is not specified and the terminal is a tty, user will be prompted to choose from a list of projects:
  
• A snippet is structured data (at the moment an xml document, but will eventually be yaml) so I've implemented set-variable to take either a path to a file or - to denote stdin. Accepting from stdin is slightly pedantic, but experienced cli users may expect it.

[alexplischke]

I'm on board with --project 👍