Migrate CassandraChatMemory to CassandraChatMemoryRepository

[markpollack]

This would allow us to have a more robust implementation than we currently have based on the ChatMemory.get(conversationId, lastn) interface that is currently deprecated.

This has been done for Jdbc in the PR - as a basis for comparison.

#2890

[linarkou]

As for now, cassandraChatMemory stores data in table with 4 columns like that:

create table ai_chat_memory (
    session_id        text,
    message_timestamp timestamp,
    assistant         text,
    user              text,
    primary key (session_id, message_timestamp)
)
    with clustering order by (message_timestamp desc);

Maybe we should change table structure similar to JdbcChatMemory?

create table ai_chat_memory (
    session_id        text,
    message_timestamp timestamp,
    type              text,
    content           text,
    primary key (session_id, message_timestamp)
)
    with clustering order by (message_timestamp desc);

It'll give us an ability to store tool/system messages, not only user/assistant.

What do you think @markpollack @michaelsembwever ?