refactor: Deprecate Spanner extensions for protobuf values

[SanjayVas]

This is now handled by the Spanner client library.

[wfa-reviewable]

This change is 

[Marco-Premier]

Reviewed 10 of 10 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @SanjayVas)

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src/test/kotlin/org/wfanet/measurement/gcloud/spanner/StatementsTest.kt line 61 at r1 (raw file):

        "EnumColumn" to cardinality.number,
        "ProtoBytesColumn" to field.toGcloudByteArray(),
        "ProtoJsonColumn" to field.toJson(),

Always wondered why the , is added to the last element? Is it a common good practice?

Code quote:

,

[SanjayVas]

Reviewable status: 9 of 10 files reviewed, 1 unresolved discussion (waiting on @Marco-Premier)

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src/test/kotlin/org/wfanet/measurement/gcloud/spanner/StatementsTest.kt line 61 at r1 (raw file):

Previously, Marco-Premier (marcopremier) wrote…

Always wondered why the , is added to the last element? Is it a common good practice?

Yes, you often want to use trailing commas on multiline structures when supported by the language. It helps to avoid merge conflicts when adding items, for example. ktfmt ensures we're consistent with trailing comma usage.

[SanjayVas]

Reviewed 1 of 1 files at r2.
Dismissed @Marco-Premier from a discussion.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @Marco-Premier)

[SanjayVas]

Reviewed 4 of 4 files at r3.
Reviewable status: 8 of 10 files reviewed, all discussions resolved (waiting on @Marco-Premier)

[SanjayVas]

Reviewed 2 of 2 files at r4.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @Marco-Premier)