* Move log's typings into grafana/ui
* Update the way context is retrieved for Loki

Major changes:

1. getLogRowContext expects row to be of LogRowModel type
2. getLogRowContext accepts generic options object, specific to a datasource of interest. limit option has been removed, and now it's a part of Loki's context query options (see below)
3. LogRowContextProvider performs two queries now. Before, it was Loki ds that performed queries in both directions when getLogRowContext.
4. Loki's getLogRowContext accepts options object of a type:

interface LokiContextQueryOptions {
    direction?: 'BACKWARD' | 'FORWARD';
    limit?: number;
}

This will enable querying in either direction independently and also slightly simplifies the way query errors are handled.

LogRowContextProvider maps the results to a Loki specific context types, basically string[][], as raw log lines are displayed in first version.