feat(ingest): Add GenericAspectTransformer

[amanda-her]

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[Khurzak]

With these changes, our aim is to create the infrastructure that will allow creating transformers to consume generic aspects.
A new GenericAspectTransformer with the following goals:

• Support GenericAspectClass instead of Aspect
• When an aspect is transformed return MetadataChangeProposalClass instead of MetadataChangeProposalWrapper

BaseTransformer has been extended to support MetadataChangeProposalClass

[asikowitz]

Thanks for your contribution to datahub! I think GenericAspectTransformer may be more appropriate as another mixin (like LegacyMCETransformer and SingleAspectTransformer, but this works as is.

[asikowitz]

We can handle both these cases by:

• Pulling the appropriate aspect out of the MCE
• Converting the MCPW into a MCP with make_mcp

[asikowitz]

I can see for simplicity why you inherited from SimpleAspectTransformer, but I think it's awkward we're essentially ignoring the main method of that class.