fix: resolve StyleCop analyzer errors and improve code compliance

[thpierce]

Fixes style issues that kept showing up in PRs (e.g. #244), and then some:

• Enable TreatWarningsAsErrors- https://github.com/search?q=repo%3Aopen-telemetry%2Fopentelemetry-dotnet-instrumentation%20TreatWarningsAsErrors&type=code
• Update StyleCop.Analyzers to v1.2.0-beta.556 to support globalization invariant mode - https://github.com/search?q=repo%3Aopen-telemetry%2Fopentelemetry-dotnet-instrumentation+StyleCop.Analyzers&type=code
• Enable XML documentation generation for main project - https://github.com/open-telemetry/opentelemetry-dotnet-instrumentation/blob/1db2ca586e1ad4a35581ea6f9d4b03d5e05bddea/src/Directory.Build.props#L7
• Remove empty AWSDistroOpenTelemetryEventSource.cs file - Initial Plugin implementation to enable AWSClientInstrumentation #16 (comment)
• Fix various code style issues:
  • CS1574: XML comment has cref attribute that could not be resolved
  • CS1587: XML comment is not placed on a valid language element
  • CS1591: Missing XML comment for publicly visible type or member
  • CS8002: Referenced assembly does not have a strong name
  • CS8602: Dereference of a possibly null reference
  • SA0001: XML comment analysis is disabled due to project configuration
  • SA1000: Keywords should be spaced correctly
  • SA1200: Using directives should be placed correctly
  • SA1204: Static elements should appear before instance elements
  • SA1216: Using static directives should be placed at the correct location
  • SA1300: Element should begin with upper-case letter
  • SA1312: Variable names should begin with lower-case letter
  • SA1316: Tuple element names should use correct casing
  • SA1402: File may only contain a single type
  • SA1414: Tuple types in signatures should have element names
  • SA1518: Use line endings correctly at end of file
  • SA1614: Element parameter documentation should have text
  • SA1616: Element return value documentation should have text
  • SA1642: Constructor summary documentation should begin with standard text
  • SA1649: File name should match first type name
• Suppress appropriate StyleCop warnings for test projects (CS8002, SA0001, SA1204, SA1300, SA1402)

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