Expose verbose category identifiers in public WiredTiger API

In order for the MongoDB server to understand and interpret a JSON event message category and level (to establish further LOGV2 mappings) we will need to make the verbose category (i.e. WT_VERB_API, WT_VERB_EVICT) and level identifiers (i.e. WT_VERBOSE_ERROR, WT_VERB_DEBUG) easily available and accessible. This would involve making the identifier definitions available in the public WiredTiger API header file.

Definition of Done:

• Export WiredTiger verbose category and level definitions via the public WiredTiger API header file.
• Update the documentation (see link in the comment) if required.