Add SPDX license header to all files

[ohltyler]

Signed-off-by: Tyler Ohlsen ohltyler@amazon.com

Description

Adds the following SPDX license header to all existing files which have the ALv2 license, as required for OpenSearch plugins:

/*
 * SPDX-License-Identifier: Apache-2.0
 *
 * The OpenSearch Contributors require contributions made to
 * this file be licensed under the Apache-2.0 license or a
 * compatible open source license.
 *
 * Modifications Copyright OpenSearch Contributors. See
 * GitHub history for details.
 */

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