add banner and icon assets

[jeffpaul]

Description of the Change

This PR adds icon and banner assets for display in the plugin update process as well as when someone views plugin details in the admin.

Alternate Designs

Keep as-is without icon and banner asset display within WP Admin.

Benefits

Ensures we appropriately display Distributor branding when available and expected within the WP Admin.

Possible Drawbacks

Additional plugin weight.

Verification Process

Manually verified via GitHub Desktop UI.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

n/a

Changelog Entry

### Added
- Plugin banner and icon assets (props @JackieKjome).

[helen]

LGTM, kind of funny the bigger banner is smaller in filesize but it's a tiny file and an even smaller difference.