Minor fixes for custom transformation

[shreyakhajanchi]

1. Added try catch block around method call to the JAR
2. Added test dependency on spanner-custom-shard
3. Generate source row from change event before any modifications to change event

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 48.28%. Comparing base (00ce8e3) to head (6b69128). Report is 11 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1722 +/- ## ============================================ + Coverage 42.20% 48.28% +6.08% + Complexity 3463 984 -2479 ============================================ Files 790 326 -464 Lines 45897 17458 -28439 Branches 4903 1738 -3165 ============================================ - Hits 19369 8429 -10940 + Misses 24951 8453 -16498 + Partials 1577 576 -1001 ``` | [Components](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1722/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/1722/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `62.92% <93.33%> (-0.59%)` | :arrow_down: | | [spanner-import-export](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1722/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/1722/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `74.14% <93.33%> (+<0.01%)` | :arrow_up: | | [spanner-live-reverse-replication](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1722/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `50.56% <ø> (ø)` | | | [spanner-bulk-migration](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1722/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `83.45% <ø> (ø)` | | | [Files](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1722?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/1722?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=) | `88.65% <93.33%> (-0.48%)` | :arrow_down: | ... and [480 files with indirect coverage changes](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1722/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform)

[Deep1998]

LGTM