Store MethodInfo when creating an ActivityDefinition

[tgrieger]

What was changed

In order to access other attributes for a given activity, the MethodInfo used to create the activity is needed. There are only certain situations where a MethodInfo is used so the new property is added but made nullable.

Why?

The main reason is to allow activity interceptors to get reflection information about the method that is being called so it can use things like Attributes to make decisions, if needed.

Checklist

1. Closes [Feature Request] Allow access to optional MethodInfo on ActivityDefinition #367

2. How was this tested:

New tests are added to verify each public Create method properly sets or does not set the MethodInfo.

3. Any docs updates needed?

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[cretz]

This looks great, only a minor suggestion

[cretz]

Thanks!