Make plugin compatible with Kotlin Script

[xgouchet]

What does this PR do?

Adds configuration DSL to easily setup the plugin when using Kotlin Script

Motivation

Fixes #154

[codecov-commenter]

Codecov Report

Merging #197 (50c35c2) into develop (48a858a) will decrease coverage by 0.45%. The diff coverage is 20.00%.

@@              Coverage Diff              @@
##             develop     #197      +/-   ##
=============================================
- Coverage      86.70%   86.25%   -0.45%     
- Complexity       177      178       +1     
=============================================
  Files             21       21              
  Lines            579      582       +3     
  Branches          76       76              
=============================================
  Hits             502      502              
- Misses            49       53       +4     
+ Partials          28       27       -1     

Files	Coverage Δ
...in/kotlin/com/datadog/gradle/plugin/DdExtension.kt	50.00% <20.00%> (-35.71%)	:arrow_down:

... and 1 file with indirect coverage changes

[KDevvv]

@0xnm when this fix will be released? It would be great to add it ASAP, and remove my hacky way to support different service name depending of variants

[0xnm]

Hello @KamilPieta! Please check version 1.12.0 of our plugin, which has change.