Add getChassisAssembly utility function

This adds a general utility function `getChassisAssembly()` to get the
list of assemblies for a given `chassisID` with the association of
`containing/containe

Add getChassisAssembly utility function

This adds a general utility function `getChassisAssembly()` to get the
list of assemblies for a given `chassisID` with the association of
`containing/contained_by` which is defined between Items [1].

This assembly starts with the following interfaces;
- xyz.openbmc_project.Inventory.Item.Panel

Tested:
- Verify the assemblies with [2]

[1] https://gerrit.openbmc.org/c/openbmc/phosphor-dbus-interfaces/+/58441
[2] https://gerrit.openbmc.org/c/openbmc/bmcweb/+/39574

Change-Id: I415185b64e8c92be5145209944a1d47c9e921359
Signed-off-by: Myung Bae <myungbae@us.ibm.com>

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