Add An Extension to add @Keep annotation to generated classes

[wuseal]

The feature is the same as #22

Add an extension via Support customizing this plugin by Extensions module

After adding this extension, it will be convenient to generate classes with @keep annotation combine with existing annotations, Code would be like this with combine Gson Annotation:

@Keep
data class Test(
    @SerializedName("a_b")
    val aB: Int = 0,
    @SerializedName("b_c")
    val bC: Int = 0,
    @SerializedName("c_d")
    val cD: Int = 0
)

[smilechenbiao]

I have completed this requirement

[wuseal]

@smilechenbiao Great! try to Create a Pull Request, and We will merge your PR after review.