Support Spring non-web services

## Is your feature request related to a problem?
I have some simple Spring Boot 2.1.x through 2.3.x applications that get messages from queues and process these using back-end services shared with other systems. These simple applications do not need a web server and are configured with:
`spring.main.web-application-type=none`

Attaching the Elastic APM Java Agent to these causes a:
`java.lang.IllegalStateException: Cannot resolve type description for javax.servlet.ServletContext`

## Describe the solution you'd like
I would like to be able to monitor these systems as I currently do for Spring Boot Servlet applications. They would likely initiate their own Traces since they're processing messages asynchronously.

## Describe alternatives you've considered
I could configure these as Servlet applications but that would defeat the purpose of keeping them small and light and would require more runtime resources, increasing the cost of operating them.