Allow SDMetrics reports to run on composite keys

[npatki]

Problem Description

In SDV 2778, we are allowing for composite keys to be specified in the metadata. Although SDV Community won't be able to model composite keys, SDV Enterprise will have this capability. Therefore, a user might have real and synthetic data with composite keys.

In this case, it should be possible for all of SDMetrics reports and metrics to accommodate composite keys in addition to single-column keys.

Expected behavior

1. SDMetrics should allow for composite keys in the multi-table metadata. To do this, a user would have to pass in a list of strings as the primary/foreign keys rather than a singular string. All the same rules as SDV apply.
2. Then, it should be possible to pass in the metadata with composite keys into the multi-table Quality Report and Diagnostic Report. Wherever it's relevant, any of the results (eg. from get_details or visualization) should report a list of columns (as the primary/foreign key) rather than individual columns.
3. Additionally, any metrics that are used by the reports should allow for composite keys. We will file these as separate issues to be done before this one.

Additional context

Consider the edge case where a user accidentally includes a list of 1 string as a primary/foreign key. In this case, it should be treated as a singular-column key, not a composite key.