release-23.2: changefeedccl: fix bug with avro encoding and zero-scale decimal cols

[blathers-crl]

Backport 1/1 commits from #118815 on behalf of @andyyang890.

/cc @cockroachdb/release

---

This patch fixes a bug where creating a changefeed that targeted
tables with a DECIMAL(n) column (i.e. zero-scale DECIMAL column),
format='avro', and diff would cause a panic.

The cause of this panic was the fact that the third-party goavro
library we use expected the JSON encoding of the schema to have
a numeric scale field for decimal types, but we omitted this
field whenever it was zero (using omitempty), which led to a
runtime type assertion failure. We've updated the field to a pointer
type in our type definition so that we can distinguish between an
unset value and a zero value.

Fixes #118647

Release note (enterprise change): Fixed a bug where creating a
changefeed that targeted tables with a DECIMAL(n) column
(i.e. zero-scale DECIMAL column), format='avro', and diff
would cause a panic.

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Release justification: low-risk, high-priority bug fix

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
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Also, please add a brief release justification to the body of your PR to justify this
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[cockroach-teamcity]

This change is 

[rharding6373]

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @andyyang890, @jayshrivastava, @nicktrav, @rafiss, and @wenyihu6)