Print number of downloaded extensions for a plugin

[tolusha]

Signed-off-by: Anatolii Bazko abazko@redhat.com

What does this PR do?

Improves the log message while downloading extensions for a plugin. It will print the number of extension is being downloaded and the total number of extensions for a plugin.

Downloading VS Code extension 1 of 1 for plugin `redhat/vscode-xml/0.7.0`
Downloading VS Code extension 1 of 2 for plugin `redhat/java/0.38.0`
Downloading VS Code extension 2 of 2 for plugin `redhat/java/0.38.0`

This PR was done in context of testing Eclipse Che with a real project (eclipse-che/che#13679)

What issues does this PR fix or reference?

eclipse-che/che#13388

[codecov]

Codecov Report

Merging #66 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #66      +/-   ##
==========================================
+ Coverage    66.6%   66.66%   +0.05%     
==========================================
  Files           6        6              
  Lines         569      570       +1     
==========================================
+ Hits          379      380       +1     
  Misses        165      165              
  Partials       25       25

Impacted Files	Coverage Δ
brokers/unified/vscode/broker.go	74.07% <100%> (+0.13%)	⬆️

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[rhopp]

#13388 is not in the endgame plan AFAIK, so this PR will have to wait until Che 7.0 is released.

[sleshchenko]

The same changes are already merged in #80