Add support for providing additional labels in CaptureError and CaptureException agent's public API

[SergeyKleyman]

Currently CaptureError and CaptureException .NET agent 's public API inherit Context, which in particular includes labels, from the corresponding transaction and there's no way to provide additional labels specific to the error and not to the whole transaction. For example, Node.js and Python agents do support providing additional labels in their capture-error public API.

Requested at https://discuss.elastic.co/t/how-to-use-distributed-tracing-in-with-elastic-apm-in-non-net-core-application/197356/5

[gregkalapos]

Implemented in #982.