Add custom request header decoder API to Kestrel (#23233)

...ef/Microsoft.AspNetCore.Server.Kestrel.Core.netcoreapp.cs

src/Servers/Kestrel/Core/src/Internal/ConfigurationReader.cs

...s/Kestrel/Core/src/Internal/Http/HttpHeaders.Generated.cs

src/Servers/Kestrel/Core/src/Internal/Http/HttpProtocol.cs

...vers/Kestrel/Core/src/Internal/Http/HttpRequestHeaders.cs

...Kestrel/Core/src/Internal/Infrastructure/HttpUtilities.cs

src/Servers/Kestrel/Core/src/KestrelConfigurationLoader.cs

src/Servers/Kestrel/Core/src/KestrelServer.cs

src/Servers/Kestrel/Core/src/KestrelServerOptions.cs

src/Servers/Kestrel/Core/test/HttpRequestHeadersTests.cs

src/Servers/Kestrel/Core/test/KestrelServerOptionsTests.cs

src/Servers/Kestrel/Core/test/UTF8Decoding.cs

...s/Kestrel/Kestrel/test/KestrelConfigurationLoaderTests.cs

...estrel/perf/Kestrel.Performance/BytesToStringBenchmark.cs

src/Servers/Kestrel/shared/KnownHeaders.cs

...l/test/InMemory.FunctionalTests/Http2/Http2StreamTests.cs

...trel/test/InMemory.FunctionalTests/Http2/Http2TestBase.cs

...trel/test/InMemory.FunctionalTests/Http3/Http3TestBase.cs

...ers/Kestrel/test/InMemory.FunctionalTests/RequestTests.cs

src/Shared/ServerInfrastructure/StringUtilities.cs