feature/query-helpers

[boyswan]

This PR contains a handful of updates/changes:

• Make metadata field naming convention consistent base_uri, part_uri

• Refactor rmrk::config::Config to allow better composition. Split Config trait into individual functions with relevant trait bounds.

• Remove cargo flags mintable, equippable as they were actually redundant. Compiled wasm contract size is the same with/without them - the only difference is in the rmrk crate dependency size at build-time. It might be better to reserve these flags for future configurable backends (psp34, uniques, etc) where conditional compilation may be required.

• Add rmrk::query::Query trait, for ui-based helpers. Tried a few different approaches but ran into various limitations - this feels the most versatile for now. Fetching children directly would lead to gas-limits quickly being hit, hence the Token type containing IDs. Not 100% satisfied with having two fields (pending & accepted) for assets/children, however this was the cleanest way to avoid introducing new duplicate types. The reason some functions are cross-contract builders is due to the impl potentially not existing on a child contract, so need to be able to directly handle the error otherwise the call will trap. Currently no tests for these, as it'll be much easier with e2e tests once we update to ink 4.0.

• Add getter methods required for Query functions get_asset, get_pending_token_assets, get_pending_children, get_accepted_children

[Maar-io]

LGTM