Enhance Repository Mixin to provide a repository getter

[bajtos]

Example usage:

// app setup
app.repository(TodoRepository);

// in place where we need to access the repo directly
const todoRepo = await app.getRepository<TodoRepository>(TodoRepository.name);

// if TS supports this pattern, then the following would provide even better UX:
const todoRepo = await app.getRepository(TodoRepository);

Loosely related: #744

Acceptance Criteria

• Create the getter function
• Add TSDocs
• Update examples to use this new approach instead of ctx.get
• Update loopback.io documents