Sql Integration, New Integration README-TEMPLATE and Solution Refinement

[KnibbsyMan]

This PR contains the following:

• Introduces new README-TEMPLATE.md that should be included in all Integrations. CONTRIBUTING.md and REAME.md have also been updated to reflect this.
• Ports SQL Activities from Elsa Core with the following differences:
  • Includes a bug fix to close: #SqlQuery activity loop reference error, SQL dynamic parameter execution did not obtain dynamic value elsa-core#6614
  • Keyword 'Variables.' has changed to 'Variable.' when using Sql expressions to match the singular wording, as with 'Input.' and 'Output.'
  • Adds REAME for Sql Integration.
  • Note: A follow up PR will be raised to remove sql activities from core.
• Projects have now been placed into group folders to help organise current and future integrations.
• Aligns Directory.Build.props with Elsa-Core's setup and re-points Project/Repo URLs to Elsa-Integrations.
• Renames 'EntityFrameworkCore' to 'EFCore' for the agent packages to minimis the risk of errors caused by exceeding 260 chars in file path length.
• Update all Nuget Packages, Point to Elsa 3.6.0-preview.2935 and Point Elsa Studio to 3.6.0-preview.1018.
• Move Agents Project from Elsa Studio.

[sfmskywalker]

Awesome! 🚀