[Feature][ios] Allow to define custom task

[tuyen-vuduc]

At the moment, the lib doesn't allow to change the name of the background task on iOS, hence only one background task is allowed.

This PR will allow us as developers to define as many tasks as we desire and with any unique names we want.

There is an extra step to let iOS know about these names in our AppDeledgate.cs.

WorkmanagerPlugin.registerTask(withIdentifier: "task-identifier")

Readme is updated with instructions.

[ened]

@tuyen-vuduc hi & sorry for not coming back to this PR for such a long time.

To be honest I don't fully understand it yet, perhaps the change set is too big.

From my understanding:
To make unique refresh/processing tasks work in the background, developers will need to add all their task identifiers to the .plist file first, right. So instead of the single entry that we use currently, multiple are needed.
This may be a useful breaking change.

.registerTaskWithIdentifier - is supposed to be called when the App starts in AppDelegate, do we need to add warnings that this should ONLY be called in applicationDidFinishLaunch:withOptions ?

[tuyen-vuduc]

@ened Yeah, that's right.

1. We need to put any task names we want in info.plist
2. We need to register in applicationDidFinishLaunch:withOptions

In my last two projects, we need more than one tasks with different names. And I think it'll be more natural to define the name we want, not the name hard-coded. That's why I create this PR.

[ened]

Maybe we could split this PR.
There are changes in iOS example setup, example file itself (the new nesting due to the scrollview prevents efficient review) and the good change itself. Possible?

[clragon]

are there any updates on this? having multiple tasks on iOS would be great.

[ened]

@tuyen-vuduc any chance you could pick this up again?

[tuyen-vuduc]

@ened Yeah, I am about to finish this within the week.

[tuyen-vuduc]

@ened Could you check why the UT fails on CI/CD check?

[ened]

@ened Could you check why the UT fails on CI/CD check?

Can you please install flutter 2.10 on your machine, run flutter pub upgrade and push the changes? I had similar warnings earlier.

[ened]

hi @tuyen-vuduc, does https://stackoverflow.com/a/58224050/375209 help with the error you currently showing in the integration test log?

[ened]

Looking good. Let's get the docs right and sort the conflict. Then happy to merge.

[tuyen-vuduc]

@ened is it ok for merging?

[tuyen-vuduc]

@ened How are you? Are you planning to merge this PR?