chore: Enable Dependabot for development dependencies

[Flash0ver]

fixes #5280

Enable Dependabot for development dependencies:

• add root, but only explicitly allow PackageReference from the root ./Directory.Build.props file ... all sub-directories (benchmarks, samples, src, test) have their dedicated rule set
• add benchmarks, and group "BenchmarkDotNet*" packages into a single PR
• add src, but only for development dependencies ... does not include any other NuGet package, that would result in changing a transitive dependency for the consumer of the published package, which could be a breaking change in some scenarios ... we update these manually with the next major release of our Sentry SDK packages

See also https://docs.github.com/en/code-security/reference/supply-chain-security/dependabot-options-reference

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.11%. Comparing base (8ddab0e) to head (e7d2722).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5281      +/-   ##
==========================================
+ Coverage   74.09%   74.11%   +0.01%     
==========================================
  Files         508      508              
  Lines       18320    18320              
  Branches     3584     3584              
==========================================
+ Hits        13575    13577       +2     
+ Misses       3872     3871       -1     
+ Partials      873      872       -1     

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