support SpringBootServletInitializer customization

how to customize the `SpringBootServletInitializer` injected by `BootInitializerClassInjector`?
for now, to apply a custom boot `ApplicationListener`, i ended up using `META-INF/spring.factories` but i would have prefered to use `SpringApplicationBuilder.listeners()`.

if this is already possible please consider this just as a question, if not then it is a feature request.

n.b. because this is no bug report i did not stick to the issue template.
also, i asked this in slack multiple times over a period of something like two weeks prior to opening this issue. unfortunately, no reply (yet).

Start by tracing BootInitializerClassInjector and the injected SpringBootServletInitializer, then inspect how SpringApplicationBuilder.listeners() is configured. The issue names no files or tests; done would require a decided, documented way to customize the initializer or confirm that the requested listener configuration is already supported.

Written by the indexing model from the issue text.