Add library model for Guava's Closer.register

[lazaroclapp]

This is a follow up to #628 and our support for @ParametricNullness in NullAway 0.9.9.
It turns out that com.google.common.io.Closer annotates it's register method as:

  @CanIgnoreReturnValue
  @ParametricNullness
  public <C extends @Nullable Closeable> C register(@ParametricNullness C closeable) {...}

Where the return is only really @Nullable if the argument is @Nullable (it is the same reference).

This PR adds a Library Model for this method, which is equivalent to adding the @Contract("null -> null")
annotation to it.

Note that, in order for this model to be helpful, AcknowledgeLibraryModelsOfAnnotatedCode
must be set, which would seem to argue for making that the default.

[lazaroclapp]

Ran into this case while upgrading internally (used in tooling, so my usual sanity tests pre-release missed it 😅 )

[coveralls]

Pull Request Test Coverage Report for Build #901

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.001%) to 92.49%

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Totals
Change from base Build #900:	0.001%
Covered Lines:	4803
Relevant Lines:	5193

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💛 - Coveralls

[msridhar]

Note that, in order for this model to be helpful, AcknowledgeLibraryModelsOfAnnotatedCode
must be set, which would seem to argue for making that the default.

Maybe we should just get rid of the flag entirely? I'm not sure how likely it is that a user will have a library model for annotated code and will prefer the annotations in the code. I'm also fine with turning the flag on by default if we want to preserve the option.

[lazaroclapp]

Note that, in order for this model to be helpful, AcknowledgeLibraryModelsOfAnnotatedCode
must be set, which would seem to argue for making that the default.

Maybe we should just get rid of the flag entirely? I'm not sure how likely it is that a user will have a library model for annotated code and will prefer the annotations in the code. I'm also fine with turning the flag on by default if we want to preserve the option.

I think for next release we should keep the flag and just flip the default just in case. I do worry that we will forget and never remove the flag, though... in either case, will make that as a separate PR if only for better Changelog tracking 🙂