Added possibility to mention in a chat message

[MathijsVerbeeck]

Issue ref: #309

Mentions possible:

• User
• Application (bots)
• Team Tag
• Conversation (Channel, chat or Team)

[codecov-commenter]

Codecov Report

Merging #840 (0fc83bb) into dev (63545f3) will increase coverage by 1.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev     #840      +/-   ##
==========================================
+ Coverage   82.42%   83.50%   +1.08%     
==========================================
  Files         416      513      +97     
  Lines       28590    34584    +5994     
==========================================
+ Hits        23565    28879    +5314     
- Misses       5025     5705     +680     

Impacted Files	Coverage Δ
...re/Model/SharePoint/Changes/Internal/ChangeUser.cs	0.00% <0.00%> (-100.00%)	⬇️
...odel/Security/Internal/SharePointUserCollection.cs	78.78% <0.00%> (-21.22%)	⬇️
....Core/Exceptions/MicrosoftGraphServiceException.cs	80.00% <0.00%> (-20.00%)	⬇️
...PnP.Core.Admin/Model/Teams/Internal/TeamCreator.cs	76.19% <0.00%> (-14.29%)	⬇️
src/sdk/PnP.Core/Services/Core/TestManager.cs	54.94% <0.00%> (-12.96%)	⬇️
.../Model/SharePoint/Core/Internal/SharePointAdmin.cs	81.06% <0.00%> (-12.80%)	⬇️
.../SharePoint/Core/Internal/ContentTypeCollection.cs	87.67% <0.00%> (-12.33%)	⬇️
...e/Model/Teams/Internal/TeamChannelTabCollection.cs	87.92% <0.00%> (-12.08%)	⬇️
.../Model/SharePoint/Core/Internal/ListItemVersion.cs	66.66% <0.00%> (-11.12%)	⬇️
...e/Model/SharePoint/Core/Internal/ListCollection.cs	82.12% <0.00%> (-9.09%)	⬇️
... and 178 more

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[jansenbe]

Awesome work again @MathijsVerbeeck!!