EVM runtime: introduce support for `ipld::block_stat` in the Blockstore interface

[raulk]

The Blockstore interface doesn't allow us to stat a block. However, we do have an FVM syscall for this. The lack of this support makes the implementation of EXTCODESIZE inefficient, as it needs to fetch the block data merely to report its size.

[Stebalien]

• We may need to rejigger costs if we do that as we currently charge to fetch the block on open.
• In terms of performance, this likely isn't a huge issue. If we call EXTCODESIZE, we're probably going to read the block immediately afterwards.

[maciejwitowski]

@raulk @Stebalien is it needed for M2.1?

[Stebalien]

I don't think so?

[raulk]

We may need to rejigger costs if we do that as we currently charge to fetch the block on open.

Blockstores have a GetSize method, but I don't think the FFI exposes it.

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Agree it's not worth it, happy to punt this to M2.2 or beyond.