feat(api7): support API-level label selector

[bzp2010]

Description

The current API7 backend uses an implementation that pulls completely and performs label filtering locally. This can be inefficient when the number of potential resources is high.
This PR moves label filtering to the API7 API level, where the server performs resource filtering and only returns resources that match the labeling rules.

It supports the following resource types:

• Consumer
• SSL

Resource types that will not be supported (These resources do not have a labels field):

• Global Rule
• Plugin Metadata

Resources that may be supported later:

• Service: Right now, its not supported for labels based filtering, we will add support for it when the server starts supporting it.

Even though services do not perform API-level label filtering, label filtering will still work and its mode of operation will fall back to full pull and local filtering. This will be improved later.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible