Implement extension.NewExecuteCommandFuncResponse and Option methods

[jaimeteb]

An approach to #18

[codecov-io]

Codecov Report

Merging #28 (c3e511f) into master (1a2439d) will decrease coverage by 0.82%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
- Coverage   59.47%   58.65%   -0.83%     
==========================================
  Files          18       18              
  Lines         923      936      +13     
==========================================
  Hits          549      549              
- Misses        300      312      +12     
- Partials       74       75       +1     

Impacted Files	Coverage Δ
extension/server.go	42.22% <0.00%> (-3.29%)	⬇️
internal/bot/server.go	35.07% <0.00%> (ø)

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[ryancurrah]

I like this approach. Though I don't feel it solves the problem of moving most of the FSM package to the internal/ directory. I still think we need Request and Response structs for the FSM. Then we could move most of the FSM into the internal directory.