Add BeJsonSerializable extensions for objects

[vbreuss]

Draft for #2555

Add the following methods for ObjectAssertions (similar to ObjectAssertionsExtensions in FluentAssertions.Json):

public AndConstraint<TAssertions> BeJsonSerializable(string because = "", params object[] becauseArgs);
public AndConstraint<TAssertions> BeJsonSerializable<T>(string because = "", params object[] becauseArgs);
public AndConstraint<TAssertions> BeJsonSerializable<T>(Func<EquivalencyOptions<T>, EquivalencyOptions<T>> options, string because = "", params object[] becauseArgs)

These methods should assert that an object can be serialized to and deserialized from JSON and retains the values of all members.

IMPORTANT

• If the PR touches the public API, the changes have been approved in a separate issue with the "api-approved" label.
• The code complies with the Coding Guidelines for C#.
• The changes are covered by unit tests which follow the Arrange-Act-Assert syntax and the naming conventions such as is used in these tests.
• If the PR adds a feature or fixes a bug, please update the release notes with a functional description that explains what the change means to consumers of this library, which are published on the website.
• If the PR changes the public API the changes needs to be included by running AcceptApiChanges.ps1 or AcceptApiChanges.sh.
• If the PR affects the documentation, please include your changes in this pull request so the documentation will appear on the website.
  • Please also run ./build.sh --target spellcheck or .\build.ps1 --target spellcheck before pushing and check the good outcome

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[coveralls]

Pull Request Test Coverage Report for Build 7521248985

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 97.43%

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Totals
Change from base Build 7520600368:	-0.03%
Covered Lines:	11961
Relevant Lines:	12151

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💛 - Coveralls