Support customized ConfigItem and ReferenceResolver in the ConfigParser

[Nic-Ma]

Is your feature request related to a problem? Please describe.
Sub task of #3482

Would be nice to support customized ConfigItem and ReferenceResolver in the ConfigParser.
A typical use case is to convert another existing config to MONAI bundle config, like NVIDIA Clara Train config, DiNTS config, etc.

[Nic-Ma]

A draft PR: #3980.

Thanks.