[RFC][BCS/TS] Support multiple schemas

[amnn]

Currently, bcs accumulates registered types in a singleton, updated by static functions so it's only possible to have one interface for each type name. This proposal suggests adding the ability to create new instances of the bcs, each with their own types registered.

Motivation

#4878 introduces a backwards incompatible change to Sui's BCS schema (the addition of a field, deep within the structure of a Move Call Transaction). For backwards compatibility, the TypeScript SDK needs to be able to talk to both the old and new schemas. This was achieved by:

• Duplicating all the BCS type registrations between ObjectArg (the type that changed) and TransactionData (the root), through 9 type registrations.

https://github.com/MystenLabs/sui/blob/d379a70a37caf4e2acf7b9aa9cfcf6a1a278aa66/sdk/typescript/src/types/sui-bcs.ts#L316-L363

• Supplying a different root type name during serialization, based on the version advertised by the RPC Provider:

https://github.com/MystenLabs/sui/blob/d379a70a37caf4e2acf7b9aa9cfcf6a1a278aa66/sdk/typescript/src/signers/txn-data-serializers/local-txn-data-serializer.ts#L345-L349

But this is quite error-prone and cumbersome, especially if we need to manage multiple unrelated schema changes. Ideally, instead of this approach, we could define multiple schemas (or a function that returns a schema, based on RPC version), so the example above becomes:

// sui-bcs.ts
function bcsSchema(version: string): BCS {
  const bcs = new BCS();

  /* ... */

  if (version === "0.11.0" || version === "0.12.0") {
    bcs.registerEnumType('ObjectArg', {
      ImmOrOwned: 'SuiObjectRef',
      Shared_Deprecated: 'ObjectID',
    });
  } else {
    bcs.registerStructType('SharedObjectRef', {
      objectId: 'ObjectID',
      initialSharedVersion: 'u64',
    });

    bcs.registerEnumType('ObjectArg', {
      ImmOrOwned: 'SuiObjectRef',
      Shared: 'SharedObjectRef',
    });
  }

 /* ... */

  return bcs;
}

export function bcsForAPI(version: string): BCS {
  /* wrapper to the above, with caching */
}

// local-txn-data-serializer.ts
const dataBytes = bcsForAPI(version).ser('TransactionData', tx, size).toBytes();