optimize Output Cache; no API changes yet - all internal:

- unify OutputCacheEntry and FormatterEntry
- leased buffers for headers, tags, etc (dispose on way out)
- use ReadOnlySequence<byte> instead of List<byte[]> with recyclable segments
- avoid copying the payload data once fectched
- serialization tweak: use common headers (not yet listed)

# Conflicts:
#	src/Middleware/OutputCaching/src/OutputCacheEntryFormatter.cs

# Conflicts:
#	src/submodules/googletest

Author: mgravell

src/Caching/StackExchangeRedis/src/RedisOutputCacheStore.cs

@@ -21,7 +21,7 @@
 
 namespace Microsoft.Extensions.Caching.StackExchangeRedis;
 
-internal class RedisOutputCacheStore : IOutputCacheStore, IOutputCacheBufferStore, IDisposable
+internal partial class RedisOutputCacheStore : IOutputCacheStore, IOutputCacheBufferStore, IDisposable
 {
     private readonly RedisCacheOptions _options;
     private readonly ILogger _logger;