Support `Key` and `Content` Substitution Types

[ServerlessSam]

What they must do:

Key:

• The value will be a jsonpath string that must point to a single node in the source file.
• If jsonpath finds multiple nodes, an error must be thrown.
• The value determined will be the name of this key.

Content:

• The value will be a jsonpath string that must point to a single node in the source file.
• If jsonpath finds multiple nodes, an error must be thrown.
• The value determined will be the value at this node. It must be a string or int (converted into str).

I don't know if this logic can ever be used in the PathSubs.Type of source files, only destination files. This is because the substitutiuon logic needs to intraspect the file in order to complete the path, but it needs the path in order to get the file. Chicken vs egg.

Let me know thoughts ...