ISchema now inherits IProvideMetadata (#2036)

Formally it is a breaking change but practically clients shouldn't run into problems, as hardly anyone creates their own ISchema implementations preferring to inherit from Schema class.

MISC:

• GitHub Security Lab (GHSL) Vulnerability Report: GHSL-2020-231 (#2031)
• Bump Microsoft.NET.Test.Sdk from 16.7.1 to 16.8.3 (#2032)
• Add built-in analyzers (#2034)
• Retarget System.Text.Json to netcoreapp3.1 (#2035)