feature(cmx): add ability to set custom username

[squizzi]

This PR adds support for overriding the GitHubUsername value set in vendor-portal. It can be overridden in the event that the username cannot be obtained from portal either for reason of error or simply the fact that the username is not populated.

Testing

• Added unit tests to the existing tests for vm_endpoints
• Manual testing:

Removed github username from vendor-portal:

./bin/replicated vm ssh-endpoint squizzi-ubuntu
Error: no GitHub account associated with Vendor Portal user
Visit the Account Settings page in Vendor Portal to link your account
Alternatively, you can use the --username flag to specify a custom username for the endpoint

Usage:
  replicated vm ssh-endpoint VM_ID_OR_NAME [flags]

Example:
  # Get SSH endpoint for a specific VM by ID
  replicated vm ssh-endpoint aaaaa11

  # Get SSH endpoint for a specific VM by name
  replicated vm ssh-endpoint my-test-vm

  # Get SSH endpoint with a custom username
  replicated vm ssh-endpoint my-test-vm --username custom-user

Flags:
  -h, --help              help for ssh-endpoint
      --username string   Custom username to use in SSH endpoint instead of the GitHub username set in Vendor Portal

Global Flags:
      --app string     The app slug or app id to use in all calls
      --debug          Enable debug output
      --token string   The API token to use to access your app in the Vendor API

Use --username flag to override:

./bin/replicated vm ssh-endpoint squizzi-ubuntu --username "banana"
ssh://[email protected]:43157

./bin/replicated vm scp-endpoint squizzi-ubuntu --username "banana"
scp://[email protected]:43157

Readd username to portal:

./bin/replicated vm ssh-endpoint squizzi-ubuntu
ssh://[email protected]:43157