Supporting appending mesh generators with actions

[YaqiWang]

Reason

We have few cases where we'd like to use actions gathering some information and appending a mesh generator (modifier), for example adding a boundary surrounding a subdomain, to avoid adding it manually by users. Current mesh generator system design does not allow it.

Design

I am thinking to have a function

std::string MooseApp::getCurrentFinalMeshGenerator() const

which returns the final mesh generator by building a DependencyResolver inside the function like currently in MooseApp::executeMeshGenerators(). Then the action can use the returned name as the input for appending mesh generators. One caveat is that users must not set a final mesh generator manually in the input file. @permcody do you have any better suggestions?

Impact

New capability.