Aura runs as a service and Neo4j maintains full control over the configuration and upkeep of the database management system and associated plugins. Therefore with Aura you cannot customize or install, upgrade or remove any server side plugins. 
Note: Aura does come with certain standard plugins pre-installed, such as a subset of APOC core and Aura plugins. If running AuraDS it comes with GDS plugins.

If you identify a plugin that you really want to use within Aura, please share this by up-voting or creating a post in our feedback board to raise this to the attention of our product management team: https://neo4j-aura.canny.io/feature-requests.