Add nullAwayOptions to configure arbitrary NullAway options

[making]

Closes gh-55

Why

NullAway has many options that this plugin does not expose as dedicated parameters, and adding them by hand does not work:

• <arg>-XepOpt:NullAway:KnownInitializers=...</arg> in maven-compiler-plugin → error: invalid flag, because the option is only valid inside the -Xplugin:ErrorProne argument.
• <compilerArg>-Xplugin:ErrorProne -XepOpt:NullAway:KnownInitializers=...</compilerArg> → plug-in not found: ErrorProne (see the second commit).

Exposing every NullAway option as a parameter is not realistic, so an escape hatch is added.

What

1. Detect existing compiler arguments regardless of their element name

The items of compilerArgs are a plain list, so maven-compiler-plugin accepts any element name and <compilerArg> is commonly used instead of <arg>. The plugin only inspected children named arg, so an existing -Xplugin:ErrorProne argument declared with another element name was not found and a second -Xplugin:ErrorProne argument was added, which makes javac fail with plug-in not found: ErrorProne. This is the error reported in the issue, and it is reproduced by the new errorprone-arg-element-name IT.

2. nullAwayOptions

<configuration>
    <nullAwayOptions>
        <KnownInitializers>com.example.api.SomeClass.init</KnownInitializers>
        <TreatGeneratedAsUnannotated>true</TreatGeneratedAsUnannotated>
    </nullAwayOptions>
</configuration>

is passed to the compiler as -XepOpt:NullAway:KnownInitializers=com.example.api.SomeClass.init -XepOpt:NullAway:TreatGeneratedAsUnannotated=true, for both main and test compilation.

The map form is used instead of the <nullAwayOption>Key=Value</nullAwayOption> list proposed in the issue: the option name is the key, so no string parsing is needed, the values may contain commas (KnownInitializers takes a comma-separated list), and an entry can override the option of the same name that the plugin derives from the other parameters instead of emitting the option twice.

The options can also be set as nullability.nullAwayOptions.<name> Maven properties, keeping the parity with the other parameters. A <configuration> entry wins over the property of the same option name.

Option names and values must not contain whitespace, because all options are appended to a single -Xplugin:ErrorProne argument; the build fails with an explicit message otherwise.

Tests

• NullabilityLifecycleParticipantTest (new): parsing from <configuration> and from properties, precedence, trimming, validation errors.
• NullAwayArgsBuilderTest, NullabilityConfigurationTest, CompilerConfigurerTest: option emission, override of derived options, defensive copy, and the duplicated -Xplugin:ErrorProne regression.
• New ITs nullaway-options (the build only succeeds because KnownInitializers reaches NullAway -- without it the compilation fails with @NonNull field 'greeting' not initialized) and errorprone-arg-element-name.
• ./mvnw verify passes (15 ITs).

README is updated with a "Setting arbitrary NullAway options" section.

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