Add mode to restic stats

[DeepikaDixit]

Change Overview

Add mode param to specify stats mode

Pull request type

Please check the type of change your PR introduces:

• Work in Progress
• Refactoring (no functional changes, no api changes)
• Trival/Minor
• Bugfix
• Feature
• Documentation

Issues

• Get Restic stats for a given snapshot #258

Test Plan

The modes are:

• restore-size: (default) Counts the size of the restored files.
• files-by-contents: Counts total size of files, where a file is
  considered unique if it has unique contents.
• raw-data: Counts the size of blobs in the repository, regardless of
  how many files reference them.
• blobs-per-file: A combination of files-by-contents and raw-data.
• Refer to the online manual for more details about each mode.

• Manual
• Unit test
• E2E