Added sample implmentation and cross functional UT for custom transformation

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 85.71429% with 4 lines in your changes missing coverage. Please review.

Project coverage is 48.45%. Comparing base (d2f4291) to head (71ed074). Report is 17 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1732 +/- ## ============================================ + Coverage 42.26% 48.45% +6.19% + Complexity 3167 998 -2169 ============================================ Files 791 328 -463 Lines 46149 17720 -28429 Branches 4939 1770 -3169 ============================================ - Hits 19504 8586 -10918 + Misses 25052 8544 -16508 + Partials 1593 590 -1003 ``` | [Components](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1732/components?src=pr&el=components&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | Coverage Δ | | |---|---|---| | [spanner-templates](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1732/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `63.25% <85.71%> (-0.39%)` | :arrow_down: | | [spanner-import-export](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1732/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `∅ <ø> (∅)` | | | [spanner-live-forward-migration](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1732/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `74.95% <0.00%> (-0.03%)` | :arrow_down: | | [spanner-live-reverse-replication](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1732/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `51.73% <92.30%> (+0.29%)` | :arrow_up: | | [spanner-bulk-migration](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1732/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `83.14% <ø> (ø)` | | | [Files](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1732?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | Coverage Δ | | |---|---|---| | [...emplates/transform/ChangeEventTransformerDoFn.java](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1732?src=pr&el=tree&filepath=v2%2Fdatastream-to-spanner%2Fsrc%2Fmain%2Fjava%2Fcom%2Fgoogle%2Fcloud%2Fteleport%2Fv2%2Ftemplates%2Ftransform%2FChangeEventTransformerDoFn.java&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform#diff-djIvZGF0YXN0cmVhbS10by1zcGFubmVyL3NyYy9tYWluL2phdmEvY29tL2dvb2dsZS9jbG91ZC90ZWxlcG9ydC92Mi90ZW1wbGF0ZXMvdHJhbnNmb3JtL0NoYW5nZUV2ZW50VHJhbnNmb3JtZXJEb0ZuLmphdmE=) | `87.75% <0.00%> (-0.91%)` | :arrow_down: | | [...n/java/com/custom/CustomTransformationFetcher.java](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1732?src=pr&el=tree&filepath=v2%2Fspanner-custom-shard%2Fsrc%2Fmain%2Fjava%2Fcom%2Fcustom%2FCustomTransformationFetcher.java&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform#diff-djIvc3Bhbm5lci1jdXN0b20tc2hhcmQvc3JjL21haW4vamF2YS9jb20vY3VzdG9tL0N1c3RvbVRyYW5zZm9ybWF0aW9uRmV0Y2hlci5qYXZh) | `92.30% <92.30%> (ø)` | | ... and [481 files with indirect coverage changes](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1732/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform)