[Bug]: SpannerIO.Write does not support named schemas

[nielm]

What happened?

If named schemas are used in Spanner database, then SpannerIO does not know about tables created in that schema.

This leads to warnings during writes of the form:

Performance issue: Mutation references an unknown table: TABLENAME

Root cause is that SpannerIO.ReadSpannerSchema only reads information about tables in the default schema.

Issue Priority

Priority: 2 (default / most bugs should be filed as P2)

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[nielm]

.add-labels: gcp,io,spanner

[Suvrat1629]

Hey @nielm is it ok if I take a shot at this issue?