Migrate plugin/plugin.go to use github.com/mattermost/mattermost/server/public

[kaakaa]

Mattermost introduced new created go module that is for developer to use mattermost-server's public API and discretely versioned from the mattermost-server. This ticket is about migrating plugin/plugin.go to use that package.

This issue is a successor of #350.

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refs: Mattermost self-hosted changelog

Introduced the public submodule, housing the familiar model and plugin packages, but now discretely versioned from the server. It is no longer necessary to go get a particular commit hash, as Go programs and plugins can now opt-in to importing github.com/mattermost/mattermost-server/server/public and managing versions idiomatically. While this submodule has not yet shipped a v1 and will introduce breaking changes before stabilizing the API, it remains both forwards and backwards compatible with the Mattermost server itself.

refs: [MM-53695] Make use of github.com/mattermost/mattermost/server/public by hanzei · Pull Request #185 · mattermost/mattermost-plugin-starter-template

[kaakaa]

refs mattermost/mattermost-plugin-github#725