Add getAssetInfo into util function

AssetInfo is needed for many schemas like Fan, Storage etc. Using this
utility function, those AssetInfo can be obtained in one place.

This function extracts the

Add getAssetInfo into util function

AssetInfo is needed for many schemas like Fan, Storage etc. Using this
utility function, those AssetInfo can be obtained in one place.

This function extracts the following properties if available.
- Manufacturer
- Model
- PartNumber
- SerialNumber
- SparePartNumber (if requested)

Tested:
- GET Chassis, Fan, Storage etc
- Redfish Service Validator passes

Change-Id: I2e8fdab8969d24899c261cfdf601be971d0210d7
Signed-off-by: Myung Bae <myungbae@us.ibm.com>

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