Add a "encrypt" command in "activate_cli.dart" which will encrypt the existing atKeys file with the pass-phrase.

[sitaram-kalluri]

Is your feature request related to a problem? Please describe.

With reference to the issue: #604, when onboarding an atSign, a pass-phrase can be supplied with which the content in the atKeys file is encrypted.

For the existing atKeys file, add the command "encrypt" in the at_activate_cli.dart to migrate to the passPhrase protected atKeys file.

Describe the solution you'd like

The "encrypt" command should accept the following arguments:
a. passPhrase to encrypt the content
b. filePath of existing atKeys file
c. an optional argument to mention the filePath where the new encrypted atKeys file should be placed.