Update apisix configuration for get public assets endpoint

[vttranlina]

Why

epic: https://github.com/linagora/tmail-backend/issues/1027 When the Tmail JMAP server is behind Apisix, we need to update the Apisix configuration to not require authentication for requests to the Get public asset endpoint.

The unrestricted endpoint is /publicAsset/{$accountId}/{$assetId}.

Ref: https://github.com/linagora/tmail-backend/issues/1045