Rename streaming_source materialization to flink_connector

The `streaming_source` materialization name is confusing because it doesn't relate to dbt's standard `sources` concept (for existing tables)
The materialization is specifically for creating Flink connector-backed source tables (e.g. `connector='faker'`).
Existing Kafka topics should be referenced as standard dbt sources in `sources.yml`, which is not obvious to users.
So `flink_connector` seems like a better naming for the materialization.

### Proposed solution

Rename `streaming_source` to `flink_connector`. This is a breaking change.

Also document how to reference existing Kafka topics as standard dbt sources, since this is going to be a common user question.

Locate the streaming_source materialization and all references to its name, then inspect the related documentation and tests. Rename the materialization to flink_connector, update the affected references for the breaking change, and document how existing Kafka topics are declared as standard dbt sources in sources.yml.

Written by the indexing model from the issue text.