(Feature request) Add call detail for calls within XCM `Transact`

[seadanda]

Currently an XCM Transact appears in the call detail window with just the encoded call data.

It would be nice if this call data could be decoded to give the details of the call that it will execute on the target parachain. Of course the amount of work for this will scale with the number of supported chains, but maybe just supporting system parachains would already be a great start.

Without this, any referendum that includes an XCM Transact will be opaque to many users.

As an example, the referendum to Enable Coretime includes an XCM with four Transact calls which are sent to the coretime chain. These need to be separately decoded to give: configure broker set core count assign pool core start sales

[wliyongfeng]

@seadanda Thanks for this issue and sorry for my late reply. I think it will bring users much convenience when inspecting proposal calls. We didn't assign it a higher priority for several reasons:

• Technically we have to create one or multiple polkadot api instances for decoding a call on a para chain, and we also have to handle components' status by different api status, like not connected, querying, query failure, etc.
• This feature is not quite common. Usually experts care more about this, but they can copy the encoded data and decode it on polkadot apps.

We are recently planing to implement treasury spend on assethub which may have similar issues with this. We will look deep into this and implement it in a appropriate time. Thanks.

[wliyongfeng]

Related issue: #4735