Add lifecycle event provider for the start of execution phase to FlowProviders

[joshfriend]

Expected Behavior

I'd like to be able to run a dataflow action at the start of the execution phase. Intended use would be to analyze the user's build configuration and environment and report any errors by failing the build early and reporting to a telemetry endpoint. This cannot be done during configuration phase because I don't want environment variables, properties or shell commands collected/run by this analysis to be captured by the configuration cache.

Current Behavior (optional)

FlowProviders currently only provides getBuildWorkResult() which causes a dataflow action to execute at the end of a build. If no flow provider is given, the docs say that the dataflow action will be executed at an undefined point:

⚠️ If you’re not using a lifecycle event provider as an input to the dataflow action, then the exact timing when the action is executed is not defined and may change in the next version of Gradle.

Current implementation causes dataflow actions to be executed at the end of a build when no lifecycle event provider is used.

Context

We had previously implemented this as a regular task, but it is difficult to make the task always run for every build command. Adding the environment scan task as a prerequisite for all other tasks in every project eagerly creates all tasks during configuration phase and bloats the cache and slows configuration down. It is also incompatible with isolated projects.

[mlopatkin]

This feature request is in the backlog of the relevant team and is prioritized by them.

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This makes a lot of sense. There are two possible ways to implement this: execute immediately (at configuration time if not running from the configuration cache), or postpone until the execution phase actually starts. For the latter, we may expose some information about the configuration, like build finished provider does for the build result.