add termination_delay_rate option

[GwonsooLee]

Add termination_delay_rate options for BlueGreen deployment. This is helpful when users want to deploy with BlueGreen strategy but terminate instances gradually in order to keep stateful status.

If instance count is 10 and termination_delay_rate is 20, then first it provisions 10 more instances for Blue status. After all of instances are healthy, then reduce the instance count of Green status. At this time, goployer will reduce only 2 instances ( 20% of 10 instances) and check if those instances are successfully terminated.

[codecov[bot]]

Codecov Report

Merging #128 (e40da80) into main (41d1c46) will increase coverage by 0.06%. The diff coverage is 10.52%.

@@           Coverage Diff            @@
##            main    #128      +/-   ##
========================================
+ Coverage   4.89%   4.96%   +0.06%     
========================================
  Files         28      28              
  Lines       4594    4630      +36     
========================================
+ Hits         225     230       +5     
- Misses      4357    4388      +31     
  Partials      12      12              

Impacted Files	Coverage Δ
pkg/aws/ec2.go	0.00% <ø> (ø)
pkg/deployer/canary.go	1.23% <0.00%> (ø)
pkg/deployer/deployer.go	1.47% <0.00%> (-0.07%)	:arrow_down:
pkg/builder/builder.go	47.26% <85.71%> (+0.73%)	:arrow_up:

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