cdc: Omit crdb metadata in AVRO schema records

We currently unconditionally emit __crdb__ attribute containing cockroach specific
type description when emitting avro schema:
```
type avroSchemaField struct {
SchemaType avroSchemaType `json:"type"`
Name string `json:"name"`
Default *string `json:"default"`
Metadata string `json:"__crdb__,omitempty"`
```
We should add an option to changefeed to omit this metadata field when emitting schema definition.
The reason for this is that this metadata may change when upgrading crdb -- and because it contains
data that's not stable across upgrades, the clients (schema registry readers) may incorrectly interpret
this as a schema change.

Jira issue: CRDB-33427