Adds Encoder.Uncompressed and Encoder.TryCompress to SystemTextJson to enable compression to be managed external to the storage layer as per the Equinox DynamoStore implementation.

Used in Equinox.CosmosStore v 4.1.0 and later to facilitate:

• improved compression via use of Brotli
• better control of what to compress or not (selecting per event case, compressing events, only selectively compressing snapshots etc)
• more complete control of testing of encoding vs event generation in integration test scenarios
• simpler API for Equinox.CosmosStore (and better symmetry with DynamoStore, which makes the overall stack easier to grok)