Materialize v0.48

v0.48.0

Introduce object owners, who can manage privileges for other roles on each object in the system by adding or revoking grants. In this release, object owners have limited functionality and are assigned as follows:
- All objects that exist at the time of a new Materialize deployment (including all system objects) are owned by the mz_system role.
- All objects that predate the release are owned by the new default_owner role.
- Any new object is owned by the user who created it.
This is part of the work to enable Role-based access control (RBAC) in a future release (#11579).
Support specifying multiple roles in the GRANT ROLE and REVOKE ROLE commands.
```
-- Grant role
GRANT data_scientist TO joe, mike;

-- Revoke role
REVOKE data_scientist FROM joe, mike;
```
This is part of the work to enable Role-based access control (RBAC) in a future release (#11579).
Add mz_internal.mz_sessions to the system catalog. This table describes all active sessions in the system.

Fix a bug where subsources were created in the public schema instead of being correctly created in the same schema as the source (#17868). This resulted in confusing name resolution for users of the PostgreSQL and load generator sources.

Improve the error messages reported in mz_internal.mz_{source|sink}_status_history and mz_internal.mz_{source|sink}_statuses with more helpful pointers to troubleshoot Kafka sources and sinks (#17805). From this release, the error column reports the full error message, and other helpful suggestions are added under details.
Stop silently ignoring NULL keys in sources using ENVELOPE UPSERT (#6350). The new behavior is to throw an error when trying to query the source. To recover an errored source, you must produce a record with a NULL value and a NULL key to the topic, to force a retraction. As an example, you can use kcat to produce an empty message:
```
echo ":" | kcat -b $BROKER -t $TOPIC -Z -K: \
  -X security.protocol=SASL_SSL \
  -X sasl.mechanisms=SCRAM-SHA-256 \
  -X sasl.username=$KAFKA_USERNAME \
  -X sasl.password=$KAFKA_PASSWORD
```
Fix a bug that prevented the correct parsing of connection settings specified using the -c option (#18239).
Respect session settings even in the case where the first statement executed errors (#18317). Previously, such errors led to these settings being ignored.