Add License rule

[arturbosch]

Additional changes on top of #2077 :

I reused the current FileProcessListener extension to store the license header inside the KtFile's which is basically caching :).
Instead of introducing a Config.Location property on the config, I went with introducing a new method for the Extension interface.

interface Extension {
    ....
    fun init(config: Config) {
        // implement for setup code
    }
    @OptIn(UnstableApi::class)
    fun init(context: SetupContext) {
        // implement for setup code
    }
}

I've marked the new SetupContext as Unstable with the new @RequiresOptIn api. This way we can add some more useful methods and properties later on without needing a major version.
When we decide that SetupContext is stable, we can remove the fun init(config: Config) function from the Extension interface as SetupContext provides the Config.

/**
 * Context providing useful processing settings to initialize extensions.
 */
@UnstableApi
interface SetupContext {
    /**
     * All config locations which where used to create [config].
     */
    val configUris: Collection<URI>

    /**
     * Configuration which is used to setup detekt.
     */
    val config: Config
}

ProcessingSettings was the ideal candidate to implement SetupContext as it has all the useful information from the cli.

[codecov-io]

Codecov Report

Merging #2429 into master will increase coverage by 0.03%.
The diff coverage is 86.56%.

@@             Coverage Diff              @@
##             master    #2429      +/-   ##
============================================
+ Coverage     82.83%   82.87%   +0.03%     
- Complexity     2166     2182      +16     
============================================
  Files           356      358       +2     
  Lines          6136     6194      +58     
  Branches       1119     1130      +11     
============================================
+ Hits           5083     5133      +50     
- Misses          478      480       +2     
- Partials        575      581       +6

Impacted Files	Coverage Δ	Complexity Δ
.../detekt/rules/documentation/EndOfSentenceFormat.kt	95.23% <ø> (ø)	10 <0> (ø)	⬇️
...otlin/io/gitlab/arturbosch/detekt/api/Extension.kt	100% <ø> (ø)	0 <0> (ø)	⬇️
...itlab/arturbosch/detekt/core/ProcessingSettings.kt	91.04% <100%> (+0.41%)	19 <2> (+1)	⬆️
.../io/gitlab/arturbosch/detekt/cli/runners/Runner.kt	92.3% <100%> (ø)	12 <0> (ø)	⬇️
...sch/detekt/rules/providers/CommentSmellProvider.kt	100% <100%> (ø)	2 <0> (ø)	⬇️
...kt/rules/documentation/AbsentOrWrongFileLicense.kt	100% <100%> (ø)	9 <9> (?)
...b/arturbosch/detekt/cli/config/ConfigValidators.kt	76.92% <100%> (+4.19%)	0 <0> (ø)	⬇️
...lab/arturbosch/detekt/core/FileProcessorLocator.kt	100% <100%> (ø)	5 <0> (ø)	⬇️
...in/io/gitlab/arturbosch/detekt/cli/OutputFacade.kt	94.44% <100%> (+0.32%)	6 <0> (ø)	⬇️
...ules/documentation/LicenceHeaderLoaderExtension.kt	75% <75%> (ø)	6 <6> (?)
... and 4 more

---

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[dector]

Thanks, @arturbosch for reopening this! Should I add more tests?

[arturbosch]

@dector that sounds awesome to me!
Maybe also some real world testing on your project if this rule does what you expect it to. Thx!

[dector]

@arturbosch Battle-tested this rule on Mozilla Fenix app with the default template file. Works like a charm and it's so freaking fast! 😍

[arturbosch]

Thanks for testing! If you realize a testcase is missing, feel free to open a new PR.

[dector]

🎉 thanks @arturbosch