Add possibility to get the ServicePrincipal via IAppManager

[mloitzl]

@jansenbe See: #1096 (comment)

[codecov-commenter]

Codecov Report

Base: 82.42% // Head: 84.21% // Increases project coverage by +1.79% 🎉

Coverage data is based on head (0f4392c) compared to base (63545f3).
Patch has no changes to coverable lines.

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1116      +/-   ##
==========================================
+ Coverage   82.42%   84.21%   +1.79%     
==========================================
  Files         416      608     +192     
  Lines       28590    41565   +12975     
==========================================
+ Hits        23565    35005   +11440     
- Misses       5025     6560    +1535     

Impacted Files	Coverage Δ
...re/Model/SharePoint/Changes/Internal/ChangeUser.cs	0.00% <0.00%> (-100.00%)	⬇️
...k/PnP.Core/Model/Teams/Internal/TeamChatMessage.cs	72.34% <0.00%> (-27.66%)	⬇️
...odel/Security/Internal/SharePointUserCollection.cs	78.78% <0.00%> (-21.22%)	⬇️
...ore.Auth/Utilities/ApplicationBuilderExtensions.cs	43.90% <0.00%> (-18.17%)	⬇️
.../SharePoint/Core/Internal/ContentTypeCollection.cs	84.69% <0.00%> (-15.31%)	⬇️
src/sdk/PnP.Core/Services/Core/TestManager.cs	53.19% <0.00%> (-14.71%)	⬇️
...PnP.Core.Admin/Model/Teams/Internal/TeamCreator.cs	76.19% <0.00%> (-14.29%)	⬇️
...e/Model/Teams/Internal/TeamChannelTabCollection.cs	87.92% <0.00%> (-12.08%)	⬇️
...k/PnP.Core/Model/SharePoint/Core/Internal/Field.cs	81.60% <0.00%> (-11.25%)	⬇️
.../Model/SharePoint/Core/Internal/ListItemVersion.cs	66.66% <0.00%> (-11.12%)	⬇️
... and 319 more

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