[Feature] Add "fast" AttributeValue factory methods to avoid Builder allocations and wire into DDB Enhanced Client

[RanVaknin]

Summary

This PR adds a codegen customization that generates per member static factory methods (createS, createN, createM, etc.) on union shapes, scoped exclusively to DynamoDB's AttributeValue, and wires those factories into the DDB Enhanced Client's WRITE path converters.

Wiring into the READ path (JSON unmarshalling) would require changes to the JSON protocol layer and is out of scope.

This feature branch consists of:

• Part [1/2] - Generate "Fast" AttributeValue construction methods to circumvent builder pattern #7039 (codegen + factory generation)
• Part [2/2] -Refactor Enhanced Client to use createX methods instead of builder #7073 (Enhanced Client wiring)

Context

AttributeValue construction and allocations is a substantial performance factor for the DDB client and consequently the DDB Enhanced client accounting for 1 to 15% overall CPU time (and on the HUGE payload size it accounted for 50% of overall Memory Allocations).

AttributeValue construction is a measurable cost on the DDB Enhanced Client write path. Each call to AttributeValue.builder().s(value).build() performs a 3 allocations:
1. builder() allocates a BuilderImpl and an EnumSet to track which union member is set
2. .s(value) assigns the value and runs handleUnionValueChange (EnumSet add + iterator)
3. .build() allocates the final immutable AttributeValue, copying fields from the builder

The createX factories eliminate steps 1 and 2 entirely.

Results:

Throughput (ops/s)

Payload	Master	CreateX	Improvement
TINY	27,398	30,063	+9.7%
SMALL	23,346	26,066	+11.7%
HUGE	8,115	10,582	+30.4%
HUGE_FLAT	11,227	13,739	+22.4%

Allocations (B/op)

Payload	Master	CreateX	Reduction
TINY	46,256	45,872	−384 B (−0.8%)
SMALL	48,292	47,472	−820 B (−1.7%)
HUGE	93,152	75,696	−17,456 B (−18.7%)
HUGE_FLAT	71,660	62,988	−8,672 B (−12.1%)

Implementaion

The codegen (AwsServiceModel.java) now supports a generateDirectUnionConstructors customization, a list of shape names for which to emit direct factories. When enabled for a shape, it generates:

A shared constant for the null/unset case:

private static final AttributeValue UNSET_INSTANCE = new AttributeValue(
    Type.UNKNOWN_TO_SDK_VERSION, null, null, null,
    DefaultSdkAutoConstructList.getInstance(), ...,
    DefaultSdkAutoConstructMap.getInstance(), ..., null, null);

A private all-args constructor that assigns every field directly (no builder, no EnumSet):

private AttributeValue(Type type, String s, String n, SdkBytes b, ...) {
    this.type = type;
    this.s = s;
    this.n = n;
    ...
}

A public static factory per union member , slightly different for scalars and for collection types (collections need a copy since they are mutable)

public static AttributeValue createS(String s) {
    if (s == null) {
        return UNSET_INSTANCE;
    }
    return new AttributeValue(Type.S, s, null, null, <sentinels>, null, null);
}

The flag is enabled for DynamoDB and DynamoDB Streams only (the sole union in both models is AttributeValue).