Enhance PluginWebHostBuilder for usage

[lukas-kd]

Current behaviour
Currently you can only call "Build" on the PluginWebHostBuilder. So you cant configure or override something.

Feature request

1. Make Services configurable through PluginWebHostBuilder
2. Make configuration configurable through PluginWebHostBuilder
3. Make logging configurable through PluginWebHostBuilder
4. Add RunDefaultAsync for the PluginWebHostBuilder
5. Create a TestHost for UnitTesting / IntegrationTesting

[fb-smit]

We should refine those requests into specific use cases and then maybe split them into a separate issue each, depending on the size, complexity or dependencies.

some ideas which came to mind so far:

• check if we can put them into a plugin on their own

Logging

• one should be able to configure different sinks which the logs can be sent to
  • check serilog sink packages and how we could make them configurable or integrate them

Configuration

• which file endings?
• order of application
• additional providers (e.g. remote, azure, vaults, etc..)