Introduce LauncherExecutionRequest

[marcphilipp]

Overview

The new Launcher.execute(LauncherExecutionRequest) method paves the
road for adding additional parameters to test execution without having
to add additional overloads of execute(). For example, for #1880
we will likely need to introduce a CancellationToken (as drafted
in #4709). Instead of having to add two new overloads, such changes will
then be easy because only LauncherExecutionRequest will need to be
changed.

LauncherExecutionRequestBuilder provides a fluent API for constructing
execution requests starting either with a TestPlan or a
LauncherDiscoveryRequest. In addition,
LauncherDiscoveryRequestBuilder.forExecution() is a convenience method
to create an execution request from a discovery request.

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Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes

[sormuras]

The following would be canonical, but also a mouthful:

	public LauncherExecutionRequestBuilder toLauncherExecutionRequestBuilder() {

I think, that forExecution is a good-enough abbreviation here.

[sormuras]

Another variant could be:

public LauncherExecutionRequest toLauncherExecutionRequest(UnaryOperator<LauncherExecutionRequestBuilder> operator) {
  return operator.apply(LauncherExecutionRequestBuilder.request(build())).build();
}

[marcphilipp]

I've added this to our list of topics for the next team meeting. 👍