Add support for external init annotations in constructors

[lazaroclapp]

NullAway has existing support for -XepOpt:NullAway:ExternalInitAnnotations=... as configuration option.
Before this PR, that option allows listing a set of class-level annotations with the following semantics:

A list of annotations for classes that are "externally initialized." Tools like the Cassandra Object Mapper do their own field initialization of objects with a certain annotation (like @Table), after invoking the zero-argument constructor. For any class annotated with an external-init annotation, we don't check that the zero-arg constructor initializes all non-null fields.

This PR extends that configuration option to also allow annotations that are added directly to the zero-arguments
constructor of such an externally initialized class. The reason for this is that it's often desired to document why
the empty constructor exists, and doing so at the declaration site for the constructor makes code more readable than
doing so at the class declaration level.

The canonical example here is GSON serialization, which requires classes to have a private zero-arguments constructor
which is called for deserialization, in addition to their normal initializing constructors.

As a follow up to this PR landing, we will update the Wiki docs accordingly.

[coveralls]

Pull Request Test Coverage Report for Build #1045

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 11 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.002%) to 93.02%

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Files with Coverage Reduction	New Missed Lines	%
../nullaway/src/main/java/com/uber/nullaway/NullAway.java	11	96.12%

Totals
Change from base Build #1044:	0.002%
Covered Lines:	5277
Relevant Lines:	5673

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💛 - Coveralls

[msridhar]

LGTM

[lazaroclapp]

Docs updated at: https://github.com/uber/NullAway/wiki/Configuration#external-init-annotations