[pallet-nfts, pallet_uniques] - Expose private structs (#6087)

# Description

This PR exposes pallet_nfts and pallet_uniques structs, so other pallets
can access storage to use it for extending nft functionalities.

In pallet uniques it also exposes collection and asset metadata storage
as they are private.
 
## Integration

This integration allows nfts and uniques extension pallets to use then
private - now public structs to retrieve and parse storage from
pallet_nfts. We are building cross-chain NFT pallet and in order to
transfer collection that houses multiple NFT owners we need to manually
remove NFTs and Collections from storage without signers. We would also
like to refund deposits on origin chain and we were unable to as struct
data was private.

We have built cross-chain pallet that allows to send nfts or collections
between two pallets in abstract way without having to look which pallet
parachain (If nfts or uniques) implements.

## Review Notes

Code exposes private structs to public structs. No breaking change.

Build runs fine, tests are also ok.
<img width="468" alt="screen1"
src="https://github.com/user-attachments/assets/f31f60b5-390c-4497-b46b-59dd561204ae">
<img width="664" alt="screen2"
src="https://github.com/user-attachments/assets/7e2aa71c-3bc4-49a9-8afc-47d4f45f4359">
<img width="598" alt="screen3"
src="https://github.com/user-attachments/assets/10f4e427-858f-460d-8644-f5750494edb0">



PR is tied with following issue:
Closes #5959 

# Checklist

* [x] My PR includes a detailed description as outlined in the
"Description" and its two subsections above.
* [x] My PR follows the [labeling requirements](

https://github.com/paritytech/polkadot-sdk/blob/master/docs/contributor/CONTRIBUTING.md#Process
) of this project (at minimum one label for `T` required)
* External contributors: ask maintainers to put the right label on your
PR.
* [ ] I have made corresponding changes to the documentation (if
applicable)
* [ ] I have added tests that prove my fix is effective or that my
feature works (if applicable)

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Co-authored-by: Branislav Kontur <[email protected]>
Co-authored-by: command-bot <>