Add plan for callback-based block creation API

Design doc for a new `lch_block_create_with_callbacks` entry point that lets
C applications supply table rows through a cell-pull callback instead of
materializing CSV files. Schema continues to come from `config.toml`; mode
is per-table (omit `source` to make a table callback-backed). Values are
exchanged via a tagged union (NULL / STRING / NUMBER / TRUE / FALSE), and
end-of-table is signalled by an `LCH_END_OF_TABLE` return from the
callback. The plan covers the C ABI, the Rust adapter, the per-table
dispatch in `State::compute`, and the verification strategy.

Signed-off-by: Lars Erik Wik <lars.erik.wik@northern.tech>

Author: larsewi