E2E testing missing `login` test

[JeyJeyGao]

What is the areas you experience the issue in?

Notation CLI

What is not working as expected?

Notation CLI E2E testing missing test cases for login command.

To solve the issue:

• Merge refactor: use oras-credentials-go for credential management #654
• Add credential store support for E2E test framework
• Add E2E test cases for the command

What did you expect to happen?

login E2E test cases is required.

How can we reproduce it?

Run make e2e

Describe your environment

Linux x64

What is the version of your Notation CLI or Notation Library?

Notation RC5