Enable nullable ref types

[AArnott]

This enables the C# 8 feature called "nullable reference types" that helps us internally as well as our users to avoid NullReferenceException and otherwise understand where we may accept or return null values in our API.

[sharwell]

I'm only part way through but found some comments.

[codecov-io]

Codecov Report

Merging #357 into master will decrease coverage by 0.5%.
The diff coverage is 79.8%.

@@            Coverage Diff             @@
##           master     #357      +/-   ##
==========================================
- Coverage   87.26%   86.76%   -0.51%     
==========================================
  Files          37       38       +1     
  Lines        2490     2524      +34     
  Branches      493      506      +13     
==========================================
+ Hits         2173     2190      +17     
- Misses        232      243      +11     
- Partials       85       91       +6

Impacted Files	Coverage Δ
src/StreamJsonRpc/WebSocketMessageHandler.cs	100% <ø> (ø)	⬆️
src/StreamJsonRpc/Exceptions/RemoteRpcException.cs	33.33% <ø> (ø)	⬆️
...treamJsonRpc/Exceptions/ConnectionLostException.cs	50% <ø> (ø)	⬆️
.../StreamJsonRpc/Exceptions/BadRpcHeaderException.cs	33.33% <ø> (ø)	⬆️
src/StreamJsonRpc/JsonRpcEventSource.cs	7.31% <0%> (ø)	⬆️
...sonRpc/Exceptions/RemoteMethodNotFoundException.cs	36.36% <0%> (-20.78%)	⬇️
src/StreamJsonRpc/NullabilityAttributes.cs	0% <0%> (ø)
src/StreamJsonRpc/HeaderDelimitedMessageHandler.cs	82.24% <100%> (+0.21%)	⬆️
src/StreamJsonRpc/Exceptions/LocalRpcException.cs	50% <100%> (ø)	⬆️
...mJsonRpc/EventArgs/JsonRpcDisconnectedEventArgs.cs	88.23% <100%> (ø)	⬆️
... and 23 more

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