Added MetadataSpannerConfig class for generating SpannerConfig for ac…

[nuggetwheat]

This is a follow-up change to the quick fix that was implemented in #25108. It is a more robust solution that makes the code less brittle so that a similar config issue doesn't crop up again in the future. For details of the problem and why this solution is necessary, see #25108 (comment).

[github-actions]

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[github-actions]

Reminder, please take a look at this pr: @lukecwik@pabloem

[Abacn]

created #25383 for tracking. this PR would close it.

[Abacn]

Sorry for the delay. Had some comments below.

[Abacn]

Thanks, LGTM

[Abacn]

please avoid rebase unless merge conflict. this mixes review history. tests already run on feature branch merged onto master

[Abacn]

retest this please

[Abacn]

looks like jenkins not triggering. will try again

[Abacn]

test failure Java_Examples_Dataflow_Java17 unrelated. merging for now. Thanks!