add ability to start task

[MaratKarimov21]

Summary

startTask mutation was added

How it works

Test plan

List of steps to manually test introduced functionality:

• Open GraphiQL App
• Make request using schema:

  query {
    me: {
      id
      name
    }
  }

• ...

Review notes

While reviewing pull-request (especially when it's your pull-request),
please make sure that:

• you understand what problem is solved by PR and how is it solved
• new tests are in place, no redundant tests
• DB schema changes reflect new migrations
• newly introduced DB fields have indexes and constraints
• there are no missed files (migrations, view templates)
• required ENV variables added and described in .env.example and added to Heroku
• associated Heroku review app works correctly with introduced changes

Deploy notes

Notes regarding deployment the contained body of work.
These should note any db migrations, ENV variables, services, scripts, etc.

References

• Resolves #### Issue
• GitHub Pull Request ####