Ensure we respect OTEL instrumentation/signal configuration

[Mpdreamz]

This ensures we respect the OTEL_DOTNET_AUTO_INSTRUMENTATION_ENABLED, OTEL_DOTNET_AUTO_{SIGNAL}_INSTRUMENTATION_ENABLED and OTEL_DOTNET_AUTO_{SIGNAL}_{INSTRUMENTATION}_INSTRUMENTATION_ENABLED family of environment variables.

It follows the cascading rules as defined here: https://github.com/open-telemetry/opentelemetry-dotnet-instrumentation/blob/main/docs/config.md#instrumentations

Renamed ELASTIC_OTEL_ENABLE_ELASTIC_DEFAULTS => ELASTIC_OTEL_DEFAULTS_ENABLED to fall more in line with other OTEL environment variable names

On top of that we now support opting in to signals & instrumentations through IConfiguration.

This now also lays the groundwork to enable/disable individual instrumentations.

I also took this opportunity to refactor the way we set ConfigSource a bit.

[stevejgordon]

LGTM