[FEATURE] Add GH workflow which checks for license headers

[Yury-Fridlyand]

Is your feature request related to a problem?

Developers sometime forget to add a license header to new files. This is not detected automatically.

What solution would you like?

An automated GH workflow which will check this

What alternatives have you considered?

N/A

Do you have any additional context?

Copy implementation from another repos.

[dai-chen]

I re-label this as a bug because I recall it was enforced before. Thanks!

[Yury-Fridlyand]

Required by https://github.com/opensearch-project/opensearch-plugins/issues/138

[acarbonetto]

Note: Some OpenSearch java files have different licence formats.

Files impacted

• core/src/main/java/org/opensearch/sql/expression/aggregation/StdDevAggregator.java
• core/src/main/java/org/opensearch/sql/expression/aggregation/VarianceAggregator.java
• core/src/test/java/org/opensearch/sql/expression/aggregation/StdDevAggregatorTest.java
• opensearch/src/main/java/org/opensearch/sql/opensearch/response/agg/CompositeAggregationParser.java
• opensearch/src/main/java/org/opensearch/sql/opensearch/response/agg/FilterParser.java
• opensearch/src/main/java/org/opensearch/sql/opensearch/response/agg/MetricParser.java
• opensearch/src/main/java/org/opensearch/sql/opensearch/response/agg/MetricParserHelper.java
• opensearch/src/main/java/org/opensearch/sql/opensearch/response/agg/NoBucketAggregationParser.java
• opensearch/src/main/java/org/opensearch/sql/opensearch/response/agg/OpenSearchAggregationResponseParser.java
• opensearch/src/main/java/org/opensearch/sql/opensearch/response/agg/SingleValueParser.java
• opensearch/src/main/java/org/opensearch/sql/opensearch/response/agg/StatsParser.java
• opensearch/src/main/java/org/opensearch/sql/opensearch/response/agg/Utils.java

Each should be investigated and fixed if possible.