source-postgres: Add use_schema_inference feature flag

The implementation of this flag is a little bit tricky for SQL
CDC connectors because the JSON schema generation is ultimately
handed off to the generic `sqlcapture` package, so I've opted to
add a property to the discovered tables indicating whether the
generated JSON schema ought to request schema inference.

source-postgres/discovery.go

@@ -50,6 +50,9 @@ func (db *postgresDatabase) DiscoverTables(ctx context.Context) (map[sqlcapture.
 			// We want to exclude the watermarks table from the output bindings, but we still discover it
 			table.OmitBinding = true
 		}
+		if db.featureFlags["use_schema_inference"] {
+			table.UseSchemaInference = true
+		}
 		tableMap[streamID] = table
 	}
 	for _, column := range columns {

source-postgres/main.go

@@ -118,7 +118,11 @@ type advancedConfig struct {
 	FeatureFlags          string   `json:"feature_flags,omitempty" jsonschema:"title=Feature Flags,description=This property is intended for Estuary internal use. You should only modify this field as directed by Estuary support."`
 }
 
-var featureFlagDefaults = map[string]bool{}
+var featureFlagDefaults = map[string]bool{
+	// When set, discovered collection schemas will request that schema inference be
+	// used _in addition to_ the full column/types discovery we already do.
+	"use_schema_inference": false,
+}
 
 // Validate checks that the configuration possesses all required properties.
 func (c *Config) Validate() error {

sqlcapture/discovery.go

@@ -192,6 +192,9 @@ func DiscoverCatalog(ctx context.Context, db Database) ([]*pc.Response_Discovere
 				{Ref: "#" + anchor},
 			},
 		}
+		if table.UseSchemaInference {
+			schema.Extras = map[string]any{"x-infer-schema": true}
+		}
 
 		var rawSchema, err = schema.MarshalJSON()
 		if err != nil {

sqlcapture/interface.go

@@ -214,6 +214,8 @@ type DiscoveryInfo struct {
 	BaseTable   bool                  // True if the table type is 'BASE TABLE' and false for views or other not-physical-table entities.
 	OmitBinding bool                  // True if the table should be omitted from discovery catalog generation.
 
+	UseSchemaInference bool // True if generated JSON schemas for this table should request schema inference.
+
 	// UnpredictableKeyOrdering will be true when the connector is unable to guarantee
 	// (for a particular table) that serialized RowKey values will accurately reproduce
 	// (when compared bytewise lexicographically) the database sort ordering of the same