Support `META-INF/spring.factories` merging in singlejar tool

[guw]

Description of the feature request:

The singlejar tool has special support for some Spring files but lacks support for merging META-INF/spring.factories files. The challenge for merging these files is that it is a properties file and entries for the same key needs to be merged into a list (separated by comma).

Which category does this issue belong to?

Java Rules

What underlying problem are you trying to solve with this feature?

Merging jars with multiple such files.

Which operating system are you running Bazel on?

No response

What is the output of bazel info release?

7.2.1

Have you found anything relevant by searching the web?

• StackOverflow post.