Make PropertyFetcher public and place it in OpenTelemetry.Api

[AlexeiScherbakov]

1. PropertyFetcher is used as copy-paste in many opentelemetry projects:

• https://github.com/open-telemetry/opentelemetry-dotnet/blob/master/src/OpenTelemetry.Instrumentation.GrpcNetClient/Implementation/PropertyFetcher.cs
• https://github.com/open-telemetry/opentelemetry-dotnet/blob/master/src/OpenTelemetry.Instrumentation.AspNetCore/Implementation/PropertyFetcher.cs
• https://github.com/open-telemetry/opentelemetry-dotnet/blob/master/src/OpenTelemetry.Instrumentation.SqlClient/Implementation/PropertyFetcher.cs

2. It very usefull for other Diagnostics.

I think that it should be public and located in OpenTelemetry.Api assembly. Perhaps it should be placed in the .NET Core framework code?

[cijothomas]

We can make it public and move it to https://github.com/open-telemetry/opentelemetry-dotnet/tree/master/src/OpenTelemetry/Instrumentation, and have all instrumentation get it from the common place.

[cijothomas]

Currently these instrumentations depend on SDK project, instead of the API project, as it has to do some conversion from legacy to new activity, which requires ActivitySourceAdapter, which is in SDK...