Define functions in system_keyspace that read from the internal cdc
tables and construct the data into internal cdc data structures.

Read the CDC stream metadata from the internal system tables, and store
it in the cdc metadata data structures.

The metadata is stored in the tables as diffs which is more storage
efficient, but when in-memory we store it as full stream sets for each
timestamp. This is more useful because we need to be able to find a
stream given timestamp and token.

the get_stream method is relevant only for vnode-based keyspaces. next
we will introduce a new method to get a stream in a tablets-based
keyspace. prepare for this by renaming get_stream to get_vnode_stream.

When choosing a CDC stream to generate CDC log writes to, if the
keyspace uses tablets, we need to choose a stream according to the
relevant metadata which is specific to tablets-enabled keyspaces.

We define the method get_tablet_stream that given a table, write
timestamp, and token, returns the stream that the log entry should
be written to.

The method works by looking up the stream metadata of the table, then
finding the relevant stream set by timestamp, and finally finding
the stream that covers the token range that contains the token.

This helper functions receives two sets of streams and constructs their
difference - closed and opened streams.

Define two new virtual tables in system keyspace: cdc_timestamps and
cdc_streams. They expose the internal cdc metadata for tablets-enabled
keyspace to be consumed by users consuming the CDC log.

cdc_timestamps lists all timestamps for a table where a stream change
occured.

cdc_streams list additionally the current streams sets for each
table and timestamp, as well as difference - closed and opened streams -
from the previous stream set.

on tablet split/merge finalization, generate a new CDC timestamp and
stream set for the table with a new stream for each tablet in the new
tablet map, in order to maintain synchronization of the CDC streams with
the tablets.

We pick a new timestamp for the streams with a small delay into the
future so that all nodes can learn about the new streams in time, in the
same way it's done for vnodes.

the new timestamp and streams are published by adding a mutation to the
cdc_streams_history table that contains the timestamp and the sets of
closed and opened streams from the current timestamp.

Allow to create CDC tables in a tablets-enabled keyspace when all nodes
in the cluster support the cdc_with_tablets feature.

Fixes scylladb#22576

Introduce basic tests creating CDC tables in tablets-enabled keyspaces,
verifying we can create and drop CDC tables, write and consume CDC log
entries, and consume the log while splitting streams.

update cdc-related tests in test/cqlpy for cdc with tablets.

* test_cdc_log_entries_use_cdc_streams: this test depends on the
  implementation of the cdc tables, which is different for tablets, so
  it's changed to run for both vnodes and tablets keyspaces, and we add
  the implementation for tablets.

* some cdc-related are unskipped for tablets so they will be run with
  both tablets and vnodes keyspaces. these are tests where the
  implementation may be different between tablets and vnodes and we want
  to have converage of both.

* other cdc-related tests do not depend on the implementation
  differences between tablets and vnodes, so we can just enable them to
  run with the default configuration. previously they were disabled for
  tablets keyspaces because it wasn't supported, so now we remove this.

previously the test set tablets to disabled because cdc wasn't supported
with tablets. now we can change this to use the default to enable it to
run with either tablets or vnodes.

add_cdc and drop_cdc were skipped because CDC wasn't supported with
tablets. now that CDC is supported with tablets we should unskip it.

Now that CDC is enabled for tablets-based keyspaces, update the docs and
added explanations about the differences.