Revert "Revert "api_core: Make PageIterator.item_to_value public. (#4702)""

[theacodes]

Reverts #4731

Do not merge until api_core 0.2.0 is being prepped for release.

Context:

This is done to prevent a release-the-world scenario in order to release bigquery, spanner, and datastore. It will be re-reverted before the next minor release of api_core.

[dhermes]

@jonparrott After #4750 goes in (and this gets rebased on top of it) I'd like to add another commit to make sure that datastore and spanner depend on google-api-core >= 0.1.5.dev1 (I'm on the fence on whether BigQuery should depend on this version, since it's a test-only change).

UPDATE: Merged #4750 and sent the commit.

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[theacodes]

I have added an alias property for the private method. This allows us to maintain compatibility even for some older clients. We can remove it at a later date.

I will merge this once CI is green.

[theacodes]

nvm, I'm going to release the world anyway, I'm gonna remove the alias.