perf(bigquery): [T2-4] avoid defensive HashMap copy for ImmutableMap in InsertAllRequest.RowToInsert

[lqiu96]

Summary & Bottleneck

• In java-bigquery/google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/InsertAllRequest.java (RowToInsert constructor), every non-ImmutableMap row passed to RowToInsert.of(...) or Builder.addRow(...) is defensively copied into new HashMap<>(content) and wrapped in Collections.unmodifiableMap(...), even when content is empty or when the caller has just constructed a single-use per-row Map<String, Object> solely for streaming ingestion.
• For an insertAll batch of $R$ rows and $C$ columns, copying a single-use HashMap into a second HashMap per row allocates $R$ extra HashMap instances, $R$ extra entry table arrays, and $R \times C$ duplicate HashMap$Node objects.

Changes

• Updated InsertAllRequest.RowToInsert (java-bigquery/google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/InsertAllRequest.java) to return ImmutableMap.of() when content.isEmpty() instead of allocating a new HashMap and UnmodifiableMap wrapper.
• Added RowToInsert.ofWithoutCopy(String id, Map<String, ?> content), RowToInsert.ofWithoutCopy(Map<String, ?> content), Builder.addRowWithoutCopy(String id, Map<String, ?> content), and Builder.addRowWithoutCopy(Map<String, ?> content) so callers constructing single-use row maps can wrap them directly in Collections.unmodifiableMap(content) (or ImmutableMap.copyOf(content) when already an ImmutableMap) without a defensive HashMap copy.
• Added unit tests in java-bigquery/google-cloud-bigquery/src/test/java/com/google/cloud/bigquery/InsertAllRequestTest.java covering RowToInsert.ofWithoutCopy and Builder.addRowWithoutCopy.

Expected Impact

• Eliminates $R$ HashMap instances, $R$ entry table arrays, and $R \times C$ HashMap$Node allocations per insertAll batch when callers use ofWithoutCopy or addRowWithoutCopy, and eliminates map/wrapper allocations for empty row maps.

Verification

• Verified with mvn test -pl java-bigquery/google-cloud-bigquery -Dtest=InsertAllRequestTest,BigQueryImplTest (157 tests, 0 failures).