Adds an experimental store/load add to config bag (#2654)

## Motivation and Context
- store multiple objects of the same type in the bag
- obtain mutable access to items in the bag

## Description
This adds `store_append` to enable storing multiple items of the same
type in the bag and `get_mut`, `get_mut_or_else` and
`get_mut_or_default` to reduce clones when possible

## Testing
UTs

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