feat(nextcloud): Add low-level helpers for WebDavResponse

[provokateurin]

Necessary refactor for #718.
Extension helpers are still there to offload the small part of logic that is useful for consumers (while the rest was just proxy variables).

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.

Project coverage is 28.71%. Comparing base (548176b) to head (efc8823).

Files with missing lines	Patch %	Lines
.../src/api/webdav/utils/webdav_response_helpers.dart	0.00%	9 Missing ⚠️

Additional details and impacted files

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##           test/nextcloud/all-props-parsed    #2561      +/-   ##
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Files with missing lines	Coverage Δ
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[Leptopoda]

I was also thinking a lot about removing the webdav file abstraction and rather have something based on package:file in a separate repo.

Is there a way we can do this without breaking the api?
We should deprecate the webdav file abstractions and give devs time to migrate (given that this is the most used code of the nextcloud package).

[provokateurin]

I was also thinking a lot about removing the webdav file abstraction and rather have something based on package:file in a separate repo.

Sounds good, but the nextcloud package wouldn't know about this abstraction, right?

Is there a way we can do this without breaking the api?
We should deprecate the webdav file abstractions and give devs time to migrate (given that this is the most used code of the nextcloud package).

I need to get rid of this for CalDAV/CardDAV support, so if we deprecate it now we will have to wait a lot longer until we can support those as well.
While deprecating it for now and removing it later would be nice, I think it is fine since there is an easy path to migrate.

[Leptopoda]

Sounds good, but the nextcloud package wouldn't know about this abstraction, right?

For the idea I head yes.

I need to get rid of this for CalDAV/CardDAV support, so if we deprecate it now we will have to wait a lot longer until we can support those as well.

Then just duplicate the code and implement the new low level api separately.
You can also keep the low level api internally for now and just base CalDav and CardDav on it, deprecating the WebdavFile at a later point.