Add JSON, XML mock perf tests for azure_core

[heaths]

Uses a mock transport to more accurately test the pipeline and deserialization.

[Copilot]

Pull Request Overview

Adds mock JSON and XML performance tests using a mock transport to isolate and measure pipeline and (de)serialization overhead.

• Introduces perf test entry point and registers mock_json (and conditional mock_xml) tests.
• Adds mock data models and test implementations for JSON and XML formats.
• Adds pipeline configuration for running perf tests via Azure DevOps YAML definitions.

Reviewed Changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
sdk/core/azure_core/perf/perf.rs	Adds perf test launcher registering mock tests.
sdk/core/azure_core/perf/mock/json.rs	Implements JSON mock perf test including pipeline and serialization.
sdk/core/azure_core/perf/mock/xml.rs	Implements XML mock perf test (behind xml feature) mirroring JSON version.
sdk/core/azure_core/perf/mock.rs	Defines shared mock list/data model types used by both tests.
sdk/core/azure_core/perf.yml	Adds scheduled Azure DevOps pipeline config for perf runs.
sdk/core/azure_core/perf-tests.yml	Declares perf test matrix (JSON enabled, XML commented).
sdk/core/azure_core/Cargo.toml	Registers the perf harness-less test target.

Comments suppressed due to low confidence (1)

sdk/core/azure_core/perf/mock.rs:1

• [nitpick] These public data model types lack doc comments; add brief descriptions to clarify structure (e.g., List represents a paged container of items, ListItemProperties contains metadata like etag and timestamps) to aid reuse and future maintenance.

// Copyright (c) Microsoft Corporation. All rights reserved.

[heaths]

See Azure/azure-sdk-for-go#25440.