Add Secret class to avoid logging secrets

[vlovgr]

Introduces Secret[A] which can wrap values to avoid secret configuration values in log output.

scala> import ciris._, ciris.refined._, eu.timepit.refined.types.string._
import ciris._
import ciris.refined._
import eu.timepit.refined.types.string._

scala> env[Secret[NonEmptyString]]("DATABASE_PASSWORD")
res0: ciris.ConfigValue[ciris.Secret[eu.timepit.refined.types.string.NonEmptyString]] = ConfigValue(Right(Secret(***)))

[codecov]

Codecov Report

Merging #73 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
+ Coverage   97.74%   97.78%   +0.03%     
==========================================
  Files          31       33       +2     
  Lines         444      452       +8     
  Branches        3        3              
==========================================
+ Hits          434      442       +8     
  Misses         10       10

Impacted Files	Coverage Δ
.../main/scala/ciris/readers/CirisConfigReaders.scala	100% <100%> (ø)
...ules/core/shared/src/main/scala/ciris/Secret.scala	100% <100%> (ø)

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