Feature/unified plugin Enabled flag

[asdacap]

• Add Enabled property to INethermindPlugin.
  • Replace IInitializationPlugin.ShouldRunSteps.
  • Replace IConsensusWrapperPlugin.Enabled.
  • Replace IConsensusPlugin.SealEngineType check.
• To allow plugins to know if it should be enabled, they are given access to IConfigs and ChainSpec from their constructor.
• Plugins that return false on Enabled is immediately disposed and not added as part of plugin list. No need to check if it enabled manually in Init as that won't be called..
• IInitializationPlugin now have a GetSteps which return the step explicitly instead of implicit checking assembly.
• Only one IConsensusPlugin can be enabled now.

Types of changes

What types of changes does your code introduce?

• New feature (a non-breaking change that adds functionality)

Testing

Requires testing

• Yes
• No

If yes, did you write tests?

• Yes
• No

Notes on testing

• Testing...