Support .net core 3 (#167)

target .net core 3.1 instead of .net core 2.1

upgrade .net core sdk to 3.1

use `paket` and `dotnet mergenupkg` as local tool

bump version

correct tag

release v0.4.2

release v0.4.1

update changelog

bump version to 0.4.0-alpha3

bump version

bump version as alpha

support json format on flat schema, reenable json tests (#60)

Support json as format using a flat schema

reenable json tests:

- `proto schema with collection` test is skipped, expected to not work
- `proto schema with just fields (flat)` test is reenabled
- added scala json interop
    - sanity check scala -> json -> scala
    - scala -> json -> .net
    - .net -> json -> scala