Create and reuse a single instance of CosmosClient in Cosmos integration tests

At the moment, the Cosmos integration tests create a new instance of the CosmosClient for performing operations (such as creating collection, inserting items, etc.) on the cosmos DB. The documentation recommends creating a single instance for the lifetime of the application. Refactoring the tests to create a single instance and re-use that could be an enhancement to the existing cosmos integration tests.

Documentation Link: [Azure Cosmos DB Performance Docs](https://docs.microsoft.com/en-us/azure/cosmos-db/sql/performance-tips?tabs=trace-net-core)