Adding GCP Spanner Change Stream support for Python

[flpablo]

Adding GCP Spanner Change Stream support for python (fixes #24103).

This update enables reading from Spanner Change Streams using apache_beam.io.gcp.spanner by incorporating cross-language transforms from the Beam Java SDK. The function outputs a JSON string of com.google.cloud.spanner.DataChangeRecord, which is necessary as DataChangeRecords are not yet directly supported, and existing solutions (like PubSub) rely on this string format.

Required parameters include instance_id, database_id, project_id, changeStreamName, inclusiveStartAt, metadataDatabase, and metadataInstance.

These changes align with the existing development for ReadFromSpanner and WriteToSpannerSchema within apache_beam.io.gcp.spanner.

Added unit test for the cross-language support for Spanner Change Stream.

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[flpablo]

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R: @damccorm for label python.
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Reminder, please take a look at this pr: @damccorm @m-trieu @nielm

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Reminder, please take a look at this pr: @jrmccluskey @kennknowles @nielm

[damccorm]

@nielm could you please take a look at this one?

[github-actions]

Reminder, please take a look at this pr: @jrmccluskey @kennknowles @nielm

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[tvalentyn]

R: @nielm

[tvalentyn]

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[nielm]

Sorry for the delay in getting to this.

Can you confirm that you have manually run integration tests on this?

[nielm]

assign set of reviewers

[github-actions]

Assigning reviewers:

R: @damccorm for label python.
R: @kennknowles for label java.
R: @nielm for label spanner.

Note: If you would like to opt out of this review, comment assign to next reviewer.

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[nielm]

LGTM

[nielm]

assign to next reviewer

[nielm]

LGTM

(again in the hope that PR-bot assigns the next reviewer in the chain...)

[damccorm]

Thanks!