Add several new observable types

[MiDoCisco]

Related Issue: N/A

Description of changes:

We are populating the observables with values which are notable for correlation or display to the users. There are several values, which we think make good observables, but are not marked as such. This PR tries to add them.

These are the details:

1. Added email.subject, email.uid and message_uid as an Observable types - type_id: 39, type_id: 40 and 45.
2. Added process_entity.uid as an Observable type - type_id: 41
3. Added file_path_t as an Observable type - type_id: 42 and marked fields as this type
   • lineage dictionary attribute
   • affected_package.path object attribute
   • file.path object attribute
   • image.path object attribute
   • kernel.path object attribute
   • malware.path object attribute
   • process_entity.path object attribute
4. Added extensions/windows/reg_key_path_t as an Observable type - type_id: 43 and marked fields as this type
   • reg_key.path object attribute
   • reg_value.path object attribute
5. Added reg_value.name as an Observable type - type_id: 44

I have updated CHANGELOG.md with the same granularity of the information it already uses, but it's not a "single-line" description. Should I reduce the detail to make it single line?

Note On Compatibility

There is a failing compatibility validator, as I changed some fields from string_t to the new file_path_t and reg_key_path_t - as this seem to be the only way to mark several different fields as same observable. Since both old and new types are strings (the new one just adds semantics) I don't see how this could break type compatibility (e.g. in generated classes). But I am happy to learn about any concerns and also whether there is a better (more compatible) way to mark multiple fields with the same observable type.