Add IsolationLevel overloads to InMemoryTransactionManager

[ShadyNagy]

Add BeginTransaction(IsolationLevel) and BeginTransactionAsync(IsolationLevel) overloads to InMemoryTransactionManager that delegate to existing parameterless methods.

• Add BeginTransaction(IsolationLevel) method
• Add BeginTransactionAsync(IsolationLevel, CancellationToken) method
• Add tests for new overloads

This makes behavior consistent - transactions are already ignored by InMemory provider, so isolation levels should be ignored too instead of throwing.

Ref: dotnet/EntityFramework.Docs#5254 (@roji's comment)

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[roji]

Thanks, this really looks to me like it was just an oversight in the original implementation.

[ShadyNagy]

@roji
The SQLite.FunctionalTests failure appears to be a flaky test - it passed in the previous run without any code changes on my part.
Let me know if you need me to do anything.