Extend app registry with methods to add providers and mimetype filters

[ishank011]

This PR adds methods to register new app providers on the fly and to set and retrieve default app providers for mime types. It has a change that might break the clients using the current version. We remove the mime type filters from ListAppProvidersRequest as that is redundant and performs the same functionality as GetAppProvidersRequest.

[glpatcern]

Looks good to me: I see you simplified the filter for ListAppProvidersRequest, I had created it with the oneof form following the pattern of other similar requests, but if you think it's better like this I'm OK.

[labkode]

@ishank011 fix the proto lock conflict. Maybe you'll need to create a new proto lock file. Please document breaking changes.

[ishank011]

@labkode done