Substrate block builder

This crate provides the [`BlockBuilder`] utility and the corresponding runtime api
[`BlockBuilder`](https://docs.rs/sc-block-builder/latest/sc_block_builder/struct.BlockBuilder.html).Error

The block builder utility is used in the node as an abstraction over the runtime api to
initialize a block, to push extrinsics and to finalize a block.

License: GPL-3.0-or-later WITH Classpath-exception-2.0