* Added MySqlContext
* Added MySqlContextFactory
* Added Pomelo.EntityFrameworkCore.MySql nuget package
* Added initial MySqlContext migrations
* Added support for creating derived ElsaContexts:
- Added new protected constructor based on following post
- https://github.com/dotnet/efcore/issues/7533
* Added Sample 24 - MySql provider with migration execution
* Updated other samples that needed to define the derived ElsaContext
* Added summary to ElsaContext constructor
* Updated persistence/Elsa.Persistence.EntityFrameworkCore MD file with MySql example
