Currently all concepts are in mutation_fragment_v2.hh and
flat_mutation_reader_v2.hh. Organize concepts similar to how the v1 ones
are: move high-level consume concepts into
mutation_consumer_concepts.hh.
Instead of lengthy blurbs, switch to single-line, machine-readable
standardized (https://spdx.dev) license identifiers. The Linux kernel
switched long ago, so there is strong precedent.
Three cases are handled: AGPL-only, Apache-only, and dual licensed.
For the latter case, I chose (AGPL-3.0-or-later and Apache-2.0),
reasoning that our changes are extensive enough to apply our license.
The changes we applied mechanically with a script, except to
licenses/README.md.
Closes#9937
In the next patch we will want to use these concepts in `mutation.hh`. To
avoid pulling in the entire `flat_mutation_reader.hh` just for these,
and create a circular dependency in doing so, move them to a dedicated
header instead.
