Add RestartTask function

[chris-vest]

As per the Confluent documentation, the connectors/$name/restart endpoint only restarts the Connector, but not the Connector's tasks. There is a separate endpoint for restarting tasks by task ID.

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[codecov]

Codecov Report

Merging #9 into master will increase coverage by 0.30%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master       #9      +/-   ##
==========================================
+ Coverage   87.39%   87.70%   +0.30%     
==========================================
  Files           3        3              
  Lines         119      122       +3     
==========================================
+ Hits          104      107       +3     
  Misses          9        9              
  Partials        6        6              

Impacted Files	Coverage Δ
connectors.go	100.00% <100.00%> (ø)

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[chris-vest]

I've noticed there's another PR open which adds this function plus some other useful ones - it doesn't look like the author of that PR is going to work on it any longer. I am happy to finish it off according to your comments on his PR?