feat: Initialize Strata.Excel.Core library with export, import, and test utilities

This commit introduces the `Strata.Excel.Core` project, a .NET library built on ClosedXML for robust Excel document generation and data import.

Key features include:
- **Export:** Flexible data export to Excel, supporting custom formatting, titles, subtitles, humanized headings, and batched processing for large datasets. Includes `ExcelContentResult` and `ZipFileContentResult` for ASP.NET Core integration.
- **Import:** Utilities to easily import data from Excel worksheets into C# objects.
- **Test Utilities:** Comprehensive helpers for comparing Excel workbooks in tests, handling resource extraction, and performing load tests.
- **Build Infrastructure:** Sets up a Dockerfile for building the library, including SonarQube and Dependency-Check scanning.
- **Project Structure:** Establishes `.gitignore`, `.dockerignore`, `nuget.config`, and a `README.md` with usage instructions and versioning guidelines.

This foundational commit provides a reusable and well-tested framework for Excel operations within Strata applications.

nuget.config

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+    <config>
+        <add key="defaultPushSource" value="https://proget.ops.stratanetwork.net/nuget/nuget" />
+    </config>
+    <packageSources>
+        <add key="NuGet official package source" value="https://api.nuget.org/v3/index.json" />
+        <add key="Strata package source" value="https://proget.ops.stratanetwork.net/nuget/nuget" />
+    </packageSources>
+</configuration>