[Variant] Fix the variant shred logic (apache#10157)

# Which issue does this PR close?

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- Closes apache#10145.


# What changes are included in this PR?

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- Seperate the logic for `variant_shred` and `variant_get` by introduce
`shred` flag for row builder
- Move some logic in `Variant::as_xxx` to `type_conversion`, and these
moved code will be used when `variant_get`, `variant_shred` will use
`Variant::as_xxx`
- After the change, when shredding a variant, all `Variant::as_xx()` is
identity function now, `Variant::Int8` can only be treated as `int8`,
but not `int16`/`int32`/`int64`, etc.
- Removed the `Variant::as_f16`
- Add a test to cover that shred can/can't be shredded to some datatype
in `test_variant_type_shredded_correctly`

# Are these changes tested?

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Yes, added some tests to cover the logic

# Are there any user-facing changes?

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Yes, some `Variant::as_xx` logic have been changed.

---------

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>
Co-authored-by: Kosta Tarasov <33369833+sdf-jkl@users.noreply.github.com>

Author: 3 people

Cargo.lock

@@ -37,6 +37,7 @@ parquet-variant-json = { workspace = true }
 chrono = { workspace = true }
 uuid = { version = "1.18.0", features = ["v4"] }
 serde_json = "1.0"
+num-traits = { version = "0.2.19", default-features = false }
 
 # uuid requires the `js` feature to run on wasm
 [target.'cfg(target_arch = "wasm32")'.dependencies]