message: Add CHANGELOG entry for cosmwasm-schema
diff: - Return value from `env.write_db` and `env.remove_db` to allow error reporting.
  - Query responses are now required to contain valid JSON.
  +**cosmwasm-schema**
  +
  +- This new crate now contains the implementations for generating JSON Schema
  + files from interface types. It exposes the functions `export_schema`,
  + `export_schema_with_title`, and `schema_for`.
  +
  **cosmwasm-std**
  - Make all symbols from `cosmwasm::memory` crate internal, as those symbols are
  