Metadata-API for Schematics

### The Problem

From https://github.com/IntellectualSites/FastAsyncWorldEdit/issues/3534

The Sponge Schematic Format supports arbitrary NBT data in schematic files. Reading / writing custom metadata is currently not supported by the WorldEdit API (SchematicReader/-Writer) and requires custom NBT parsing / writing after WorldEdit is done with NBT I/O.

### A Solution

API to support writing / reading NBT data from the `Metadata` tag when reading / writing schematic files. Should potentially keep streamed schematic formats in mind.

### Alternatives

_No response_

### Anything Else?

After discussing this with Maddy, this should most likely target WE8. Also, this could be part of a broader design question regarding "Schematic holder object"s and extending Schematics (see #dev-team).

Start with the WorldEdit API's SchematicReader and SchematicWriter interfaces and the WE8 target mentioned in the issue. Review how schematic files handle the Metadata tag and consider the implications for streamed schematic formats and a possible schematic holder object. Done means the API can read and write arbitrary NBT metadata without custom parsing outside WorldEdit.

Written by the indexing model from the issue text.