Improve vary-by rules (#43257)

...ware/OutputCaching/samples/OutputCachingSample/Startup.cs

src/Middleware/OutputCaching/src/CacheVaryByRules.cs

src/Middleware/OutputCaching/src/LoggerExtensions.cs

src/Middleware/OutputCaching/src/OutputCacheKeyProvider.cs

src/Middleware/OutputCaching/src/OutputCacheMiddleware.cs

src/Middleware/OutputCaching/src/OutputCachePolicyBuilder.cs

...are/OutputCaching/src/Policies/SetCacheKeyPrefixPolicy.cs

...iddleware/OutputCaching/src/Policies/VaryByValuePolicy.cs

src/Middleware/OutputCaching/src/PublicAPI.Unshipped.txt

...dleware/OutputCaching/test/MemoryOutputCacheStoreTests.cs

...ware/OutputCaching/test/OutputCacheEntryFormatterTests.cs

...dleware/OutputCaching/test/OutputCacheKeyProviderTests.cs

...ddleware/OutputCaching/test/OutputCacheMiddlewareTests.cs

...Middleware/OutputCaching/test/OutputCachePoliciesTests.cs

...eware/OutputCaching/test/OutputCachePolicyBuilderTests.cs

...ware/OutputCaching/test/OutputCachePolicyProviderTests.cs

src/Middleware/OutputCaching/test/TestUtils.cs