[Variant] Add variant_kernels benchmark (#7944)

# Which issue does this PR close?

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- Part of #7911
- Part of  #6736
- Follow on to #7905


# Rationale for this change

I wrote benchmark some changes to the json decoder in
#7911 but they are non trivial.
To keep #7911 easier to review I
have pulled the benchmarks out to their own PR

# What changes are included in this PR?

1. Add new json  benchmark
2. Include the `variant_get` benchmark added in
#7919 by @Samyak2
# Are these changes tested?

I tested them manually and clippy CI coverage ensures they compile

# Are there any user-facing changes?

No these are only benchmarks

Author: alamb

parquet-variant-compute/Cargo.toml

@@ -41,9 +41,11 @@ name = "parquet_variant_compute"
 bench = false
 
 [dev-dependencies]
+rand = "0.9.1"
 criterion = { version = "0.6", default-features = false }
-rand = { version = "0.9.1" }
+
 
 [[bench]]
-name = "variant_get"
+name = "variant_kernels"
 harness = false
+