[Request] Allow registering package ID for other networks

## Description

Currently, the package registry is [hardcoded](https://github.com/iotaledger/identity/blob/0ab03dab26bfbd0f8aa75edbfe37484fd8d9d792/identity_iota_core/src/rebased/iota/package.rs#L25) and package-private. It would be great if this would have some public API to manipulate it.

## Motivation

We are hosting own localnets for evaluation purposes. They all have an identity package. If our application wants to support more of them, we have to decide based on the CID if we can build the IOTA client directly or need to build a client with one of our package IDs. It would be nice if we could have one modifiable package registry per application instance which we can populate as we require it.

## Requirements
Write a list of what you want this feature to do.

1. `PACKAGE_REGISTRY.push(…)`

## Are you planning to do it yourself in a pull request?

No