Add automated license header checks

[saratvemulapalli]

We have to standardize the way to develop on opensearch-project.
We should be able to have license header checks.

[ketanv3]

OpenSearch plugins' source code should include a valid SPDX license header (along with the existing license headers, if any).

Further, plugins should enable the 'licenseHeaders' check in their build.gradle file if it was previously disabled.
Example - https://github.com/opensearch-project/asynchronous-search/blob/main/build.gradle#L117

Plugin authors can also use IntelliJ's Copyright functionality to auto-generate this license header. This Copyright profile can be made a part of the plugin repo itself so that headers are consistently generated for all contributors.

[dblock]

@ketanv3 any interest in picking this up, writing it up in this repo, and helping track it across plugins?

[ketanv3]

@dblock Sure, please assign this to me.

[ketanv3]

Update: Turns out that a few plugins prefer auto-generating the license headers using plugins like Spotless. Rather than imposing these plugins to use IntelliJ's Copyright feature, letting each plugin decide what's best for them. For now, simply enabling the license header checks for such plugins.

Plugin checklist:

• alerting
• anomaly-detection
• asynchronous-search (Enabled automated license header checks asynchronous-search#17)
• data-prepper
• index-management
• job-scheduler (Enabled automated license header checks job-scheduler#36)
• k-NN (Enabled automated license header checks k-NN#41)
• opensearch-notebooks
• performance-analyzer (Enabled automated license header checks performance-analyzer#22)
• reports-scheduler
• security
• sql

[saratvemulapalli]

@ketanv3 thanks for taking care of this. You don't have to make changes in every plugin.
Let the plugins own it.

Can you document the process on how to setup the license header checks in this repository and we can take @MMakaria's help to drive it across all the plugins to make sure all plugins will follow the steps you have recommended.

[ketanv3]

@saratvemulapalli Sure! I'll raise a PR documenting this process.

[saratvemulapalli]

Closing this as documentation for setting up license headers is done.
Plugin owners can follow the process recommended in https://github.com/opensearch-project/opensearch-plugins/blob/main/HEADERS.md#automated-license-headers-checks