refactor: Split Data-related classes out into ServiceLayer.Common

[ianfnelson]

Description

Added ServiceLayer.Common project, with root namespace of ServiceLayer.
Moved data-related classes into this project, within ServiceLayer.Data
(note that EF model classes are hence in ServiceLayer.Data.Models; this namespace choice was intentional, to denote to devs that these classes represent EF models rather than pure POCOs)

Context

Allows data related functionality to be referenced from both the ServiceLayer.MESH deployable, and also future use such as from ServiceLayer.API

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.