Use enums for State and Health properties

Several utility functions return State and Health values as
std::string literals, which lacks compile-time type safety.
This can lead to typos or invalid values that are only caught
at runtime.

Convert these functions to return resource::State and
resource::Health enums instead. These enums are already
defined in resource.hpp with NLOHMANN_JSON_SERIALIZE_ENUM
mappings and are used extensively throughout the codebase.

The enums automatically serialize to the same JSON strings, so
there is no functional change. This provides compile-time type
checking.

Tested:
- No functional change.
- Verified State and Health properties serialize to identical
  JSON values.
- Redfish Service Validator: PASS: 5788, WARN: 353, FAIL: 0,
   NOT TESTED: 4878

Change-Id: I590fc3dddac6f126c26814b549d2fea961409170
Signed-off-by: Vinothkumar Shanmugavel <vinothkumars@ami.com>