That depends on what you mean by "provider", I suppose. Google is not going to be interested in doing this, I suppose a consultancy or MSP could offer this as a service if they resell Mattermost, but I don't know if Mattermost has that sort of sales network or system.
On the other hand, for enterprise plan users it would make sense to rely on Mattermost for app distribution, and they have the expertise to publish the apps and ensure they're approved in the respective stores. Would a reseller or MSP necessarily have that competency? I come from the IT world where the answer would definitely be "no", but I could see it being part of contracted out work. Who owns the app then after that contract is up?
On the other hand, for enterprise plan users it would make sense to rely on Mattermost for app distribution, and they have the expertise to publish the apps and ensure they're approved in the respective stores. Would a reseller or MSP necessarily have that competency? I come from the IT world where the answer would definitely be "no", but I could see it being part of contracted out work. Who owns the app then after that contract is up?