fix: Downgrade androidx.metrics dep to beta01 in Gradle build

[awforsythe]

refs: RUM-10762

What and why?

As explained in RUM-10762, Android builds are broken on Unity 2022 and older, due to a transitive dependency included by dd-sdk-android-rum: namely, androidx.metrics:metrics-performance. As of version 1.0.0-beta02, that library requires an Android Gradle plugin version of 8.6.0 or newer (while versions of Unity prior to Unity 6 are stuck on AGP 7.x). The prior version, 1.0.0-beta01, is compatible with AGP 7.

This PR adds a IPostGenerateGradleAndroidProject callback to Unity Android builds. That callback, implemented as DatadogGradlePostProcessor, does the following:

• Determines whether the fix needs to be applied: if running Unity 6 or later, we return here and make no changes to gradle.build.
• Reads the gradle.build file after it's been modified by EDM4U.
• Finds the section of the file where EDM4U has declared dependencies, then locates the declaration of dd-sdk-android-rum as a dependency within that section. If we can find no such declaration, we return here and make no changes.
• Checks the declaration to see if it's already been modified: if so, we return here and make no changes.
• Replaces the implementation specifier for dd-sdk-android-rum with one that explicitly excludes androidx.metrics:metrics-performance, preserving the existing version specifier for dd-sdk-android-rum
• Adds an extra implementation specifier that explicitly pulls in androidx.metrics:metrics-performance at the desired (older) version

I've also added tests to cover these cases.

The net result is that after EDM4U generates a build.gradle file containing this dependencies block:

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
// Android Resolver Dependencies Start
    implementation 'com.datadoghq:dd-sdk-android-logs:2+' // Packages/com.datadoghq.unity/Editor/DatadogDependencies.xml:10
    implementation 'com.datadoghq:dd-sdk-android-ndk:2+' // Packages/com.datadoghq.unity/Editor/DatadogDependencies.xml:12
    implementation 'com.datadoghq:dd-sdk-android-rum:2+' // Packages/com.datadoghq.unity/Editor/DatadogDependencies.xml:8
// Android Resolver Dependencies End

    constraints {
         implementation("org.jetbrains.kotlin:kotlin-stdlib-jdk8:1.8.0") {
             because("kotlin-stdlib-jdk8 is now a part of kotlin-stdlib")
         }
    }

}

...we will further modify it to:

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
// Android Resolver Dependencies Start
    implementation 'com.datadoghq:dd-sdk-android-logs:2+' // Packages/com.datadoghq.unity/Editor/DatadogDependencies.xml:10
    implementation 'com.datadoghq:dd-sdk-android-ndk:2+' // Packages/com.datadoghq.unity/Editor/DatadogDependencies.xml:12
    implementation('com.datadoghq:dd-sdk-android-rum:2+') { // Packages/com.datadoghq.unity/Editor/DatadogDependencies.xml:8
        // DatadogGradlePostProcessor: exclude the dependency on androidx.metrics:metrics-performance:1.0.0-beta02
        // Version beta02 requires Android Gradle plugin 8.6.0+, which is not supported on Unity 2022 and older
        exclude group: 'androidx.metrics', module: 'metrics-performance'
    }
    // DatadogGradlePostProcessor: Explicitly require version beta01 of the same dependency, as it works with AGP 7
    implementation 'androidx.metrics:metrics-performance:1.0.0-beta01'
// Android Resolver Dependencies End

    constraints {
         implementation("org.jetbrains.kotlin:kotlin-stdlib-jdk8:1.8.0") {
             because("kotlin-stdlib-jdk8 is now a part of kotlin-stdlib")
         }
    }

}

This unblocks Android Unity builds, at the cost of using an older version of androidx.metrics:metrics-performance.

The net effect of this fix is similar to the fix employed for dd-sdk-reactnative: it's simply a bit more complicated to execute since the build.gradle file is generated by Unity (and post-processed by EDM4U) on a per-project basis.

Review checklist

• This pull request has appropriate unit and / or integration tests

[awforsythe]

CI jobs for this PR were routinely failing due to an unrelated flaw in the CI scripts (in short: writing subprocess output to stdout asynchronously can raise BlockingIOError if output buffers become full). So I instead validated this change in a separate branch that contains fixes for those CI issues: https://gitlab.ddbuild.io/DataDog/dd-sdk-unity/-/pipelines/70106481

Bypassing checks.