Add template FieldAction for COMBINE

[igarashitm]

• MappingType: COMBINE
• Source field: some Strings, like

{
  "firstname": "foo",
  "lastname": "bar",
  "phone": "111-111-1111"
}

• Target field: String
• User input to the field action: "You have a message from ${firstname} ${lastname}, call ${phone}."
• Expected output: "You have a message from foo bar, call 111-111-1111."

FieldAction name would be 'template' or something.

[igarashitm]

Note that we haven't had any FieldAction for COMBINE and SEPARATE, may hit some pitfall to achieve it.

[igarashitm]

Maybe it's a CombineStrategy, but not sure if the strategy is configurable from UI