refactored plugin handlings

[sio4]

• added an interface Plugin for Checker and Exporter
• remove init() function and implemented Plugins() function with reflect package. Oh, this is over-engineering.
• changed register functions to be compatible with the above changes
• cleaned up and simplified exporters and checkers.

[codecov]

Codecov Report

Merging #6 (98b56b9) into main (49e8706) will increase coverage by 42.13%.
The diff coverage is 68.32%.

@@             Coverage Diff             @@
##             main       #6       +/-   ##
===========================================
+ Coverage   29.58%   71.71%   +42.13%     
===========================================
  Files          16       17        +1     
  Lines         622      693       +71     
===========================================
+ Hits          184      497      +313     
+ Misses        438      183      -255     
- Partials        0       13       +13     

Impacted Files	Coverage Δ
internal/common/gcp.go	100.00% <ø> (ø)
exporters/stackdriver.go	4.31% <7.24%> (+0.31%)	⬆️
meari/server.go	73.68% <33.33%> (+73.68%)	⬆️
cmd/bogo/main.go	35.48% <66.66%> (+35.48%)	⬆️
internal/common/context.go	88.88% <80.00%> (-11.12%)	⬇️
checks/ping.go	85.36% <80.95%> (+80.36%)	⬆️
exporters/exporters.go	82.60% <88.88%> (+36.45%)	⬆️
checks/checks.go	100.00% <100.00%> (+53.84%)	⬆️
checks/heartbeat.go	100.00% <100.00%> (+84.00%)	⬆️
exporters/stdout.go	100.00% <100.00%> (+83.33%)	⬆️
... and 17 more

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