Add @sachinpkale as an OpenSearch maintainer

I have nominated and maintainers have agreed to invite Sachin Kale
(@sachinpkale) to be a co-maintainer. Sachin has kindly accepted.

Sachin has led the design and implementation of the remote backed
storage feature in OpenSearch. This feature was introduced as
experimental in OpenSearch 2.3 and is planned for general availability
in 2.9. Some significant issues and PRs authored by Sachin for this
effort are as follows:

Feature proposal: #1968
Upload segments to remote store post refresh: #3460
Add rest endpoint for remote store restore: #3576
Add RemoteSegmentStoreDirectory to interact with remote segment store: #4020

In total, Sachin has authored 57 PRs going back to May 2022. He also
frequently reviews contributions from others and has reviewed nearly 100
PRs in the same time frame.

Signed-off-by: Andrew Ross <andrross@amazon.com>

MAINTAINERS.md

@@ -22,6 +22,7 @@ This document contains a list of maintainers in this repo. See [opensearch-proje
 | Owais Kazi               | [owaiskazi19](https://github.com/owaiskazi19)           | Amazon      |
 | Rishikesh Pasham         | [Rishikesh1159](https://github.com/Rishikesh1159)       | Amazon      |
 | Ryan Bogan               | [ryanbogan](https://github.com/ryanbogan)               | Amazon      |
+| Sachin Kale              | [sachinpkale](https://github.com/sachinpkale)           | Amazon      |
 | Sarat Vemulapalli        | [saratvemulapalli](https://github.com/saratvemulapalli) | Amazon      |
 | Shweta Thareja           | [shwetathareja](https://github.com/shwetathareja)       | Amazon      |
 | Suraj Singh              | [dreamer-89](https://github.com/dreamer-89)             | Amazon      |