improv: only allow pm.vault.get and pm.vault.set to access vault secrets in scripts

[Pranav2612000]

What does this change do?

• Updates the pm.vault export to only support get and set methods in scripts in sandbox
• Remove the layer of vaultSecrets from pm.variables

How is this done?

• For only supporting get and set options, instead of exporting pm.vault as the VariableScope of vault secrets, we export pm.vault as a new object which only supports the get and set method. These methods internally call the VariableScope methods.
• For removing vault secrets from variables, we simply remove the line adding vault secrets to the variable layer

How is this change tested

• Added/Updated unit tests for the following scenarios
  • pm.vault only has get and set methods
  • pm.variables does not include vault secrets