feat: add obfuscation utilities and precomputed DTOs

[leoromanovsky]

Motivation

The precomputed client receives server-side computed flag assignments in an obfuscated wire format. This PR adds the data transfer objects (DTOs) and utilities needed to parse and work with this format.

Changes

Utilities:

• ObfuscationUtils - MD5 hashing for flag key obfuscation lookups

DTOs:

• PrecomputedFlag - Represents a precomputed flag assignment (allocation, variation, type, value, logging config)
• PrecomputedBandit - Represents a precomputed bandit assignment (action, probability, attributes)
• PrecomputedConfigurationResponse - Wire protocol response with flags, bandits, salt, and metadata
• BanditResult - Simple container for bandit action lookup results

Exceptions:

• MissingSubjectKeyException - Thrown when subject key is required but not provided

Tests:

• ObfuscationUtilsTest - MD5 hash consistency and standard test vectors
• PrecomputedConfigurationResponseTest - Serialization round-trips, null handling, environment parsing

Decisions

• Used Jackson annotations for JSON parsing (consistent with rest of SDK)
• Made DTOs immutable with @JsonCreator constructors
• Added @JsonIgnoreProperties(ignoreUnknown = true) for forward compatibility
• Environment field supports both string and object formats for flexibility
• Base64-encoded fields are kept encoded in DTOs; decoding happens at usage site

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PR Stack

This PR is part of the precomputed client feature, split for easier review:

main
├── precomputed/1-base-cache-file
│   └─┬─ precomputed/3-storage-layer
│     │   └── precomputed/4-client
│     │       └── precomputed/5-example-app
│     │
└── 👉 precomputed/2-dtos-and-utils (this PR) ─┘

Why this structure: DTOs and utilities have no dependencies on other new code, making them independently reviewable and testable. They form the "vocabulary" that the rest of the feature uses.