Support populating Spanner source database id, instance id and a custom metadata field in Spanner CDC to Pub Sub template

[ShuranZhang]

Introduce two new options for Spanner Change Streams to Pub/Sub template.

1. includeSpannerSource: boolean option default to false.
   If this config is set to true, two new fields spannerDatabaseId and spannerInstanceId will be populated to the message data field with the associate values.

2. outputMessageData: string option default to empty.
   If this config has non-empty string value, the string value will be populated to the outputMessageMetadata field in output pub sub message body.

I verified correctness for both AVRO and JSON data formats through integration tests in SpannerChangeStreamsToPubSubIT.java.

[codecov]

Codecov Report

Attention: Patch coverage is 62.50000% with 48 lines in your changes missing coverage. Please review.

Project coverage is 43.33%. Comparing base (1e49734) to head (8205dc6).
Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
...rt/v2/transforms/WriteDataChangeRecordsToAvro.java	72.60%	17 Missing and 3 partials ⚠️
...FileFormatFactorySpannerChangeStreamsToPubSub.java	0.00%	14 Missing ⚠️
...ort/v2/templates/SpannerChangeStreamsToPubSub.java	0.00%	12 Missing ⚠️
...rt/v2/transforms/WriteDataChangeRecordsToJson.java	93.10%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1769      +/-   ##
============================================
+ Coverage     43.28%   43.33%   +0.04%     
- Complexity     3514     3515       +1     
============================================
  Files           832      832              
  Lines         48553    48647      +94     
  Branches       5222     5235      +13     
============================================
+ Hits          21016    21080      +64     
- Misses        25848    25874      +26     
- Partials       1689     1693       +4     

Components	Coverage Δ
spanner-templates	63.66% <ø> (+0.02%)	⬆️
spanner-import-export	63.83% <ø> (+0.02%)	⬆️
spanner-live-forward-migration	75.11% <ø> (ø)
spanner-live-reverse-replication	68.93% <ø> (ø)
spanner-bulk-migration	84.19% <ø> (+0.05%)	⬆️

Files with missing lines	Coverage Δ
...rt/v2/transforms/WriteDataChangeRecordsToJson.java	87.09% <93.10%> (+27.09%)	⬆️
...ort/v2/templates/SpannerChangeStreamsToPubSub.java	0.00% <0.00%> (ø)
...FileFormatFactorySpannerChangeStreamsToPubSub.java	0.00% <0.00%> (ø)
...rt/v2/transforms/WriteDataChangeRecordsToAvro.java	60.60% <72.60%> (+9.35%)	⬆️

... and 6 files with indirect coverage changes

[Abacn]

approval based on passed tests