Update the partial data state infrastructure to use pivot reports for audiences.

[techanvil]

Feature Description

The initial implementation of the partial data state infrastructure requests a report per audience in order to determine the date of the first event for each audience.

Once we have added support for pivot reports (see https://github.com/google/site-kit-wp/issues/8484), we can reduce the number of reports by making a single request for a pivot report to retrieve the data for all audiences.

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Acceptance criteria

• When retrieving the partial data state for a set of audiences, a single pivot report should be requested to retrieve the data for all the audiences, rather than an individual report per audience.

Implementation Brief

For reference see the initial implementation of *determineResourceDataAvailabilityDate() and how it calls getPartialDataReportOptions() for each audience.

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Test Coverage

QA Brief

Changelog entry

[techanvil]

Moved to Backlog until we get all the P1 issues done and/or the initial release out.

[techanvil]

Note: This will need to be reviewed for viability in light of the issues with pivot reports for audiences as described in https://github.com/google/site-kit-wp/issues/8726#issuecomment-2211014589.