Add Mattermost as ConfigType for notifications channel

[cwperks]

Description

Companion common-utils PR: opensearch-project/common-utils#853

This PR adds a new ConfigType called Mattermost to the notifications ConfigType enum. Mattermost accepts the same payload format as Slack, but different URL naming. Since OpenSearch 3.0.0, users are saying they can no longer use the "Slack" notification channel type for other slack-like communication platforms like mattermost because of the URL validation logic. This PR seeks to establish a separate channel type for those other communication platforms like Discord, RocketChat, Zulip and Mattermost

Related Issues

Part of #1048

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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