Feature: Code cleanup, harmonization and impactless optimizations

[Metal-Mighty]

Is your feature request related to a problem? Please describe.
The IoT Portal code has been piling up a small technical debt that we should handle before it gets too big. It would make the code cleaner, provide better insights on the current code quality and technical debt, and make it overall easier to work with the portal.

Describe the solution you'd like

• Task: Aggregate similar page's code in generic components #3201
• Task: Move Blazor code-behind in .blazor.cs files #3173
• Task: Separate Unit Tests in different projects #3174
• Task: Group all using lines in unique GlobalUsings.cs files in the projects #3175
• Task: Check and fix namespaces #3181
• Task: Replace CodeCov with SonarCloud (requires validation from CGI-FR org) #3172
• Task: When SonarCloud is available, fix the security issues and major bugs #3183
• Task: Harmonize the use of System.Text.Json over Newtonsoft.Json #3185

Describe alternatives you've considered
N/A

Additional context
These changes mustn't have any impact on the current code quality and behavior