feat(service): Add Mattermost service

[shivuslr41]

Describe the missing service. Please provide useful links.

• Add Mattermost support - https://mattermost.com
• This service is a kind of slack alternative.

Describe the solution you'd like
Users should be able to send messages/notifications via notify.

Potential libraries to use
official go driver -> https://github.com/mattermost/mattermost-server/blob/master/model/client4.go
reference -> https://github.com/containrrr/shoutrrr/tree/main/pkg/services/mattermost
API docs -> https://api.mattermost.com

[shivuslr41]

Hi @nikoksr, I would like to grab this one and give it a try if notify can have this service...

[shivuslr41]

@nikoksr, please feel free to close this issue and the pull request if notify is not planning to include Mattermost service.