feat: add transform_fpe benchmark test type for Format Preserving Encryption (FPE) support

[abhijeetvlokhande]

Summary

This PR adds a new transform_fpe benchmark test type to support Format Preserving Encryption (FPE) benchmarking using Vault's Transform Secrets Engine.

Changes

• Add benchmarktests/target_secret_transform_fpe.go implementing the transform_fpe test type
• Add docs/tests/secret-transform-fpe.md with configuration and usage documentation
• Add unit tests for transform_fpe
• Add an HCL fixture for config parsing coverage

Features

• Supports all three tweak sources: internal, generated, and supplied
• Automatically creates the FPE template, transformation, and role during setup
• Cleans up created resources when cleanup = true

Testing

Added unit coverage for:

• registration of transform_fpe
• HCL config parsing
• setup / target / cleanup flow against a mocked Vault API

Targeted test status:

• go test ./benchmarktests -run TransformFPETest -v passes

Repo-wide test status:

• go test ./... still only fails in the repo's existing Docker-backed tests under test-fixtures when Docker is unavailable (Cannot connect to the Docker daemon at unix:///var/run/docker.sock)

HCP validation output:

• supplied tweak: count 2240, throughput 37.154109 req/s, mean 268.279325ms, p95 288.924115ms, p99 341.030866ms, success 100%
• internal tweak: count 2229, throughput 36.985815 req/s, mean 269.681022ms, p95 279.109931ms, p99 363.232827ms, success 100%
• generated tweak: count 2289, throughput 37.958554 req/s, mean 262.850362ms, p95 272.670632ms, p99 309.930134ms, success 100%

[hashicorp-cla-app]

All committers have signed the CLA.

[abhijeetvlokhande]

Added unit coverage for transform_fpe and updated the PR description with fresh HCP validation output.