Fix for issue 20371 related to deep depencies

[fgarciacorona]

The goal of this PR is to provide a fix for issue #20371 and improve the performance of csharp runtime when dealing with deep nested dependencies of .proto files.

• The fix consists in caching the recursive calls for the search of extensions.

• A test dataset has been created with deeply nested .proto files that showcases the performance penalty.

• A set of benchmark metrics have been created to evaluate the performance impact before and after the fix.

• Unfortunately due to not being able to unload all the existing objects inside the same testcase, it is only possible to run the test before and after enabling the caching mechanism. I am open to any other idea on how to improve this.

The test results before and after the fix are the following:
Before the proposed fix:

FileDescriptor_CreateMessageWithDeepDependencies_BuildFromByteString
   Source: DescriptorsTest.cs line 40
   Duration: 2,1 min

  Message: 
  Expected: 399
  But was:  76306932


  Stack Trace: 
DescriptorsTest.FileDescriptor_CreateMessageWithDeepDependencies_BuildFromByteString() line 61
1)    at Google.Protobuf.Reflection.DescriptorsTest.FileDescriptor_CreateMessageWithDeepDependencies_BuildFromByteString() in D:\Git\protobuf\csharp\src\Google.Protobuf.Test\Reflection\DescriptorsTest.cs:line 61

  Standard Output: 
Running performance test for extension registry caching: With caching
{ }
GetAllExtensionsCount: 402
GetAllGeneratedExtensionsCount: 573
GetAllDependedExtensionsCount: 76306932
GetAllDependedExtensionsFromMessageCount: 644966683
TotalReturnedExtensionsCount: 118
w/ cache elapsed: 00:02:04.6190917

After the proposed fix:

 FileDescriptor_CreateMessageWithDeepDependencies_BuildFromByteString
   Source: DescriptorsTest.cs line 40
   Duration: 117 ms

  Standard Output: 
Running performance test for extension registry caching: With caching
{ }
GetAllExtensionsCount: 402
GetAllGeneratedExtensionsCount: 573
GetAllDependedExtensionsCount: 399
GetAllDependedExtensionsFromMessageCount: 39
TotalReturnedExtensionsCount: 118
w/ cache elapsed: 00:00:00.0700036

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[tonyliaoss]

Hello @fgarciacorona, we won't be able to start reviewing your PR until the CLA has been signed.

Also, I suspect that we won't accept the newly added test protos, but we can discuss that after we start our review process.