Add license header check/fix using spotless

[dbwiddis]

Signed-off-by: Daniel Widdis [email protected]

Description

Uses the licenseHeader feature of spotless to apply a license header.

WIthout proper headers, build will fail:

> Task :spotlessLicenseCheck FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':spotlessLicenseCheck'.
> The following files had format violations:
      src/main/java/org/opensearch/sdk/ActionListener.java
          @@ -5,7 +5,6 @@
           ·*·this·file·be·licensed·under·the·Apache-2.0·license·or·a
           ·*·compatible·open·source·license.
           ·*/
          -
           package·org.opensearch.sdk;
           
           import·org.opensearch.common.SuppressForbidden;
      src/main/java/org/opensearch/sdk/ClusterSettingsResponseHandler.java
          @@ -1,3 +1,10 @@
          +/*
          +·*·SPDX-License-Identifier:·Apache-2.0
          +·*

... snip ...

      src/main/java/org/opensearch/sdk/ExtensionsRunner.java
          @@ -4,11 +4,7 @@
           ·*·The·OpenSearch·Contributors·require·contributions·made·to
           ·*·this·file·be·licensed·under·the·Apache-2.0·license·or·a
      ... (9 more lines that didn't fit)
  Violations also present in:
      src/main/java/org/opensearch/sdk/LocalNodeResponseHandler.java
  Run './gradlew :spotlessApply' to fix these violations.

As suggested, ./gradlew spotlessApply fixes the problem (and is how all the changed files were generated).

Netty4 package is excluded from this check (reviewers: should it be?)

Issues Resolved

Fixes #50

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