chore: add identicon generation to avalanche and optimism

[kaladinlight]

• icon field is required and in the case of missing icon, avalanche and optimism weren't generating identicons as placeholder

[shapeshift-ci-bot]

:tada: This PR is included in version @shapeshiftoss/asset-service-v8.8.2 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: