feat: Add completed future for effects

[ufrshubham]

Description

This PR adds a completed future for effects which completes when the effect finishes.

Checklist

• I have followed the Contributor Guide when preparing my PR.
• 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 or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Related Issues

NA

[spydon]

Missing docs, otherwise it looks good!
Maybe we should remove the callback in V2 and just have completers everywhere?

[ufrshubham]

Missing docs, otherwise it looks good!

Added a line about this in the docs now.

Maybe we should remove the callback in V2 and just have completers everywhere?

Maybe it is fine to keep both because there is slight difference between the two. Callbacks can execute multiple times if the event repeats. On the other hand, we set the completers to null once it is triggered.