The STORAGE option is designed to hold a map of options
used for customizing storage for given keyspace.
The option is kept in a system_schema.scylla_keyspaces table.
The option is only available if the whole cluster is aware
of it - guarded by a cluster feature.
Example of the table contents:
```
cassandra@cqlsh> select * from system_schema.scylla_keyspaces;
keyspace_name | storage_options | storage_type
---------------+------------------------------------------------+--------------
ksx | {'bucket': '/tmp/xx', 'endpoint': 'localhost'} | S3
```
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
