* WIP on adding support for mattermost

Signed-off-by: Craig Perkins <cwperx@amazon.com>

* Add to SendMessageActionHelper

Signed-off-by: Craig Perkins <cwperx@amazon.com>

* Add mattermost

Signed-off-by: Craig Perkins <cwperx@amazon.com>

* Make non-default

Signed-off-by: Craig Perkins <cwperx@amazon.com>

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Signed-off-by: Craig Perkins <cwperx@amazon.com>
Co-authored-by: Hailong Cui <ihailong@amazon.com>