feat: return block value if block was passed to Honeybadger.context

[d4rky-pl]

This PR improves on the local contexts feature introduced in #541 by making Honeybadger.context return the result of the block instead of the Agent instance when local contexts are used.

This allows seamless wrapping of existing application code:

class SuperImportantPaymentProcessing
  def call
    Honeybadger.context({ tags: ['critical', 'payments'] }) do
      # bunch of things happening
      { success: true, charge: }
    rescue PaymentError => e
      Honeybadger.notify(e) # this still gets tagged!
      { success: false }
    end
  end
end

[stympy]

Thanks!