Add Authzed / SpiceDB emoji

[benvernier-sc]

Description

Adding an emoji for Authzed / SpiceDB.

Useful to mark steps that validate and apply schemas.

PR checklist

• [X] The image is 64x64 PNG image
• [X] Image is added to img-buildkite-64 directory
• [X] Image is referenced in img-buildkite-64.json file
• [X] Image is referenced in README markdown file
• [ ] The image can be hosted by Buildkite and made available in Buildkite UI