Mattermost-server: Allow searching for files through plugin API

[mattermod]

Doesn't look like we expose an api to allow searching for files programmatically. As per discussion
https://community.mattermost.com/core/pl/7rxgk5rajtb67p161uw38xmw6o and
https://community.mattermost.com/core/pl/qoxcxnmurff9mkfx98gct4ct5e. It makes sense to expose a plugin api for searching files.

models.GetFilesOptions {
    // UserIds optionally limits the files to those created by the given users.
    UserIds []string
    // ChannelIds optionally limits the files to those created in the given channels.
    ChannelIds []string
    // Since optionally limits files to those created after a specified time as Unix time in milliseconds
    Since int64
    // IncludeDeleted includes deleted files if set.
    IncludeDeleted bool
    // Page limits to the requested page of results.
    Page int
    // PerPage limits the number of results to fetch per page.
    PerPage int
    // SortBy optionally sorts the files by this field.
    SortBy:         models.CHANNEL_SORT_BY_CREATED, // "CreateAt"
    // SortDirection sorts the files in a particular order, default is ASCENDING.
    SortDirection:  models.CHANNEL_SORT_ASCENDING,
}
GetFiles(*model.GetFilesOptions) ([]*model.FileInfo, *model.AppError)

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JIRA: https://mattermost.atlassian.net/browse/MM-17912

[rv404674]

I will take it up. Can you please assign this to me.

[jasonblais]

Opening up for grabs

[ashishbhate]

@jasonblais I'll take this one