Partial verification secondary

[tanishqjasoria]

Purpose of the PR:

Implement and demonstrate the partial verification Secondary.
Addressing the issue #48

Summary of Changes:

• Update the structure of the partial metadata distributed by the Primary
• Update demo file to comply with the partial verification Secondary also
• Add function to verify the partial metadata, compling to the latest Uptane standard
• Update sample metadata to include an update for the new partial verification secondary
• Update test_secondary to perform tests for the partial verification Secondary also
  • Verifying metadata and retrieving the target fileinfo
  • Validating the image for the partial verifying secondary

Further Requirements:

The Readme files needed to be updated