feat(datagen): add Pool[T] generic type with random selection helpers

[Dylan-M]

Proposed Change

Adds the Pool[T] generic type with random selection helpers — the foundational primitive used by every subsequent datagen pool.

Part of PIPE-927 common data generation package stack. Foundation for PIPE-785, PIPE-928, PIPE-943, and the rest of the simulator stack.

Checklist

• Changes are tested
• CI has passed

[Dylan-M]

• refactor(generators): replace embedded data pools with datagen imports across apache_combined, apache_error, nginx, paloalto, winevt #156
• docs(datagen): add documentation for identity hierarchy and seed configuration #155
• feat(datagen): add Windows process, registry, and task path pools #154
• feat(datagen): add HTTP method, path, protocol, and status code pools #153
• feat(datagen): add Environment orchestration with GenerateEnvironment #152
• feat(datagen): add ApplicationIdentity with per-role app allocation #151
• feat(datagen): add ServiceIdentity with per-role service allocation #150
• feat(datagen): add GroupIdentity with built-in AD groups and membership #149
• feat(datagen): add name pools and UserIdentity generation #148
• feat(datagen): add SystemIdentity with OS/arch/role types and version pools #147
• feat(datagen): add DomainIdentity and CertAuthority generation #146
• feat(datagen): add NetworkIdentity and IP/port/MAC generation utilities #145
• feat(datagen): add mythology hostname pools and generation functions #144
• feat(datagen): add SeedConfig with per-type seed resolution #143
• feat(datagen): add Pool[T] generic type with random selection helpers #142 👈 (View in Graphite)
• main

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