exploration of .NET 5 and C# 9 features (protobuf-net#715)

* add initial test of init-only property serialization

* add reflection API checks

* test init-only on more TFMs

* add stub for record types; can't test currently

* - add net5 to the tests
- add SkipLocalsInit
- fixup one broken test (Type metadata)
- fixup one functional error (don't emit initonly on static fields)

* add net5 to the benchmarks; remove netcore30

* dummy for Utf8String

* record types; requires tweak to tuple detection

* add a shotgun smattering of [DynamicallyAccessedMembers] to help convince the linker not to cull DTOs

* build problem; check partial records

* fix 5.0 TFM

* package updates part 1

* lib updates part 2

* lib update part 3

* simplify TFM consumption

* Install 5.0.100 on AppVeyor

Co-authored-by: Nick Craver <[email protected]>

save all the files

also fix streaming directionality

Improve .proto generation output (protobuf-net#675)

* experimental support for multiple namespaces; is "opt in" - need to use new SchemaGenerationOptions API with SchemaGenerationFlags,MultipleNamespaceSupport

xref protobuf-net#647

* first stabs at service proto emit

* - make SchemaGenerationOptions simpler / mutable
- make Types an input on SchemaGenerationOptions; remove <T>/Type overloads that take SchemaGenerationOptions

Add protobuf-net.AspNetCore with input/output formatters, and show ex…

…ample usage (protobuf-net#673)

* Add protobuf-net.AspNetCore with input/output formatters, and show example usage

* nit, unnecessary using declaration

* tweak build script

* add comment on buffering

* tyop

* make ParseIfComplete more obvious

* trying to fix CI

* - remove some tabs and ConfigureAwait (@davidfowl feedback)
- remove all the wwwroot things we aren't using

* RegisterProtobufFormatters() => AddProtoBufNet() - mimic code from MvcNewtonsoftJsonOptionsExtensions etc

* supported chunked usage and file-buffering of over-large inputs

* drop a local since it will rarely be reused

* remove TryRead from the slow path

* - increase in-memory threshold to 256k
- use DI more consistently
- fix namespaces

* remove length from WriteResponseBodyAsync

* respect input/output buffer suppression (note: for input this only applies *after* the memory-buffer threshold)

* release notes

re-address GRPC 100 (3.0.1) in a way that doesn't need a new API

release-notes (and we may as well go to 3.0.1)

3.0

add IMeasuredProtoOutput<T>