[subxt] Allow transactions in recent blocks to be decoded

[jsdw]

The Vision

Subxt can already decode events, constants and storage entries, but it can't decode recent blocks. This feels like a key missing part of the API, so let's add support for this.

The Plan

• Create a "block-centric" API for subscribing to blocks and accessing the transactions, events and storage in the block.
• Allow extrinsic bytes to be dynamically decoded into some Values representing call data.
• Allow attempts to statically decode extrinsics into some tuple or struct representing call data?

Open Questions

• Can we do anything about block signatures?
• How far can we statically vs dynamically decode transactions? I think we'll look to the Events API for inspiration here and probably offer a similar interface. Like Events, we may not have the information we need to be able to decode all transactions (we might know only about a subset of pallets).

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Here is the corresponding GitHub milestone:

https://github.com/paritytech/subxt/milestone/15

[lexnv]

Patch series

• Add block-centric Storage API subxt#774
• extrinsics: Decode extrinsics from blocks subxt#929

Subxt is now capable of:

• decoding as root extrinsic: enum containing all possible extrinsics from all pallets
• decoding as pallet extrinsics: enum containing all possible extrinsics from a pallet
• decoding as extrinsic variant: struct containing just the extrinsic details of a pallet
• decode extrinsics into some dynamic Value<()> via exposing the bytes and field's shape
• exposing the address bytes of the account that signed the extrinsic if applicable
• iterating extrinsics from the block

These enhancements will significantly improve the decoding functionality of subxt and
enable more efficient analysis of extrinsics.