cli: refactor fitactivity

[muktihari]

New Feature

This PR adds new feature "reduce" to reduce record messages in a FIT file to answer this issue #394 .

This has 3 method to choose:

1. Based on GPS points using RDP Ramer-Douglas-Peucker.
2. Based on distance interval in meters
3. Based on time interval in seconds`

Refactor

How we can interact with fitactivity CLI is also refactored for more structured commands (arguably easier to access and maintain). Some commands may differ, but they should be simple to follow. cmd/fitactivity/README.md file has been updated accordingly regarding the changes.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (2979f67) to head (52cf188).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #398   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           41        41           
  Lines         3691      3691           
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  Hits          3691      3691           

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