Fix incremental table compilation failing on metadata.extraProperties

[fuzmish]

Example

This fails to compile:

config {
  type: "incremental",
  onSchemaChange: "IGNORE",
  metadata: {
    extraProperties: {
      myField: "example"
    }
  }
}

SELECT 1
// -> Unexpected property "myField", or property value type of "string" is incorrect.

Cause

verifyObjectMatchesProto(protoType, object) in common/protos/index.ts:

const proto = protoType.create(object);          // shallow copy, no type conversion
const protoCastObject = protoType.toObject(proto);
checkFields(object, protoCastObject);             // throws on typeof mismatch at any key

create() copies extraProperties as-is, without converting it to the Struct format.
toObject() then can't read it as a Struct, so protoCastObject ends up with an empty extraProperties.
checkFields finds myField present in the raw object but missing from the round-tripped one, and throws.

Fix

Convert metadata.extraProperties into the Struct wire format in IncrementalTable.verifyConfig(), before calling verifyObjectMatchesProto, so both sides of the comparison already have matching shapes.
The conversion logic already existed; it is extracted into a named, exported function and incremental_table.ts call it directly.

Test

• Added a test in core/actions/incremental_table_test.ts covering the example above.
• bazel test $(bazel query 'kind(test, //core/...)') passed.

Related: #2119