feat: Allow children of ComposedParticle to have varied lifespan

[akvus]

Description

Existing behavior: ComposedParticle takes lifespan and children as a constructor argument. At the construction time lifespan is propagated to all child particles.

New behavior: ComposedParticle has an extra optional parameter applyLifespanToChildren. If set to true then existing behavior is preserved and all child particles will be assigned the lifespan of ComposedParticle, if set to false child particles will keep their original lifespan. If a particle lifecycle ends before ComposedParticle ends then it is removed from children.
applyLifespanToChildren is set to true by default for backward compatibility.

Please note, that this is a proposal. I will update the docs and examples if the approach is approved.

Breaking Change

[x] No, this is not a breaking change.

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have read the Contributor Guide and followed the process outlined for submitting PRs.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples.

Related Issues

#1872

[spydon]

The proposal looks good to me!