pmm-admin summary must include targets from vmagent

User story:

• As a PMM user, I need to be able to get the list of targets scraped by vmagent on the client-side so that I can simplify troubleshooting of problems and do it faster

 
Note: VM in this context means VictoriaMetrics

UI: 

user run 

#pmm-admin summary  ... 

In the generated file presented file vmagent-targets.html with HTML presentation 

and  vmagent-targets.json with  json presentation of targets output  

TBC: List of sources for requested data (Tech)

note: no vmagent on pmm-server

Acceptance criteria:

• vmagent targets files (two) presented  in "support" zip . PMM client 2.12+ 
• no changes for pmm before 2.12, no VM 

How to test:

pmm-agent: make env-up, make  setup-dev, make run 

check if page can open http://127.0.0.1:42001/targets

run pmm-admin summary 

zip should contain file client/vmagent-targets.html 

file should contain information from http://127.0.0.1:42001/targets