Deprecate OwncloudClient - Helpers

[ZetaTom]

This is one of a series of pull requests which aim to replace all instances of OwnCloudClient with NextcloudClient. The reason for this change is that the newer NextcloudClient uses OkHttp, replacing the outdated Jackrabbit methods.

Specifically, this pull request implements the following:

• helper classes to ease the transition and refactoring
• general changes
• fallback mechanism to convert legacy OwnCloudClient to NextcloudClient at runtime
• see https://github.com/nextcloud/android-library/pull/1271

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• [ ] Tests written, or not not needed

[github-actions[bot]]

blue-Light-Screenshot test failed, but no output was generated. Maybe a preliminary stage failed.

[github-actions[bot]]

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[tobiasKaminsky]

Will be in https://github.com/nextcloud/android/pull/12266