feat: add options, diagnostics, URL redaction, and the pipeline context

[OmarAlJarrah]

Summary

Adds the building blocks the pipeline and policy slices depend on — all in Dexpace.Sdk.Core, with no new runtime dependencies.

• DexpaceClientOptions (+ RetryOptions, RedirectOptions) — plain configuration POCOs with sensible enterprise defaults (3 retries with capped backoff, max 20 redirects, no HTTPS→HTTP downgrade, no cross-origin header leakage). The Microsoft.Extensions.Options binding and validation will live in the DI integration package, so these stay container-agnostic and new-able.
• DexpaceDiagnostics — a single ActivitySource and Meter named Dexpace.Sdk, so OpenTelemetry / Application Insights light up with no adapters. (System.Diagnostics.DiagnosticSource is in-box on net8/net10, so no package was needed.)
• UrlRedactor — produces a log-safe URL string: strips userinfo, drops the fragment, and replaces sensitive query-parameter values with REDACTED (configurable, case-insensitive), while preserving the path. Robust for both absolute and relative URIs — it never throws and never emits an un-redacted sensitive value.
• PipelineContext — the mutable per-call state (request, response-as-it-arrives, the active Activity, the options snapshot, an attempt counter, and a typed property bag) that flows through the pipeline.

Test plan

• dotnet build -c Release is clean on net8.0 and net10.0 (warnings-as-errors + XML-doc gate)
• dotnet test -c Release passes (74 tests: 61 core, 13 serialization)

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