metadata/v15: Collect transaction extensions by versions into custom metadata

[lexnv]

This PR collects the transaction_extensions_by_version type information into the metadata V15 custom types.

This ensures the V15 contains enough information to work with multiple transaction extensions. The meaning of this field is identical to the one collected by the V16 unstable. It includes indexes of extrinsics from the signed_extension field of the extrinsic metadata.

Exposed Metadata

"custom": {
        "map": {
          "transaction_extensions_by_version": {
            "ty": 1074,
            "value": [
                  4, 0, 36, 0, 0, 0, 0, 1, 0, 0, 0, 2, 0, 0, 0, 3, 0, 0, 0, 4, 0, 0, 0, 5, 0, 0, 0, 6, 0, 0,
                  0, 7, 0, 0, 0, 8, 0, 0, 0,
              ]
          }
        }
      }

The decoded value is Decoded: {0: [0, 1, 2, 3, 4, 5, 6, 7, 8]}.

use codec::Decode;
use std::collections::BTreeMap;

fn main() {
    let value = vec![
        4, 0, 36, 0, 0, 0, 0, 1, 0, 0, 0, 2, 0, 0, 0, 3, 0, 0, 0, 4, 0, 0, 0, 5, 0, 0, 0, 6, 0, 0,
        0, 7, 0, 0, 0, 8, 0, 0, 0,
    ];
    let decoded: BTreeMap<u8, Vec<u32>> = Decode::decode(&mut &value[..]).unwrap();
    println!("Decoded: {decoded:?}");
}

cc @paritytech/subxt-team

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/12163465313
Failed job name: test-linux-stable

[gui1117]

I don't think this is enough, this encode the transaction extension indices used for each version.
But the v15 metadata extensions doesn't contains all the extensions.

If we have 2 version. v15 metadata will contains the first version only.
so we need to add in custom metadata all the other transaction extension, or just all the transaction extensions same as v16.

[lexnv]

Indeed, this will no longer work when we include new transactions in the next version.
We collect the same transaction extensions (signed extensions), but that will not be the case in the future. I'll introduce the transaction extensions as well, thanks for pointing this out 🙏

[lexnv]

Introducing a new transaction extension versioning (ie version 1) should happen after the metadata V16 is stabilized (pending on pallet view functions at the moment). Therefore this is not needed in this form, we'll revisit it at a later time