Add ephemeral attribute to main branch

[lrcouto]

Description

Adds the _EPHEMERAL attribute to the AbstractDataset class. This attribute will serve to indicate if a Dataset object is persistent or not. It is set to be False by default, and should be changed to True in case a Dataset is not meant to be persistent, as seen, for example, in MemoryDataset.

Development notes

Comments and discussion can be seen on #3520

Developer Certificate of Origin

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Checklist

• Read the contributing guidelines
• Signed off each commit with a Developer Certificate of Origin (DCO)
• Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• Updated the documentation to reflect the code changes
• Added a description of this change in the RELEASE.md file
• Added tests to cover my changes
• Checked if this change will affect Kedro-Viz, and if so, communicated that with the Viz team

[ankatiyar]

Thanks @lrcouto!