API: filtering for things

[s-kostyuk]

This pull request is related to the things filtering implementation.

The following statements are proposed:

• ApiGateway allows to perform filtering in 'get_things' method. Filter is a dictionary that contains the desired values of things. If all values in filter are the same as the values in corresponding Thing fields, then this Thing is saved to resulting list.
• Filtering is added for '/things/' path in REST API as well. Values for filtering are specified via request parameters. For example, to get all things that are placed in placement 'R1', the following request must be performed:

  curl --request GET \
  --url 'http://dpl_host:dpl_port/things/?placement=R1' \
  --header 'Authorization: your_auth_token_here'

[s-kostyuk]

Filtering by parameters is quite popular. It's described or used:

• in "Web API Design" e-book
• on Hackernoon: article
• and also here and here

So, parameter-based filtering will be implemented (actually, as initially proposed). Not a /things/?filter=placement:R1 or similar approach

[s-kostyuk]

This pull request will be closed, another implementation is pending. For future updates follow this issue: https://github.com/s-kostyuk/adpl/issues/19