add 'excludes' tag support for spring-boot-maven-plugin

[xiuyouxu]

add 'excludes' tag support for spring-boot-maven-plugin

spring-boot-maven-plugin can excludes dependencies, this pull request contains support to this tag. Dependencies in excludes will not be added to the final libs directory.

see first-party/jib-spring-boot-extension-maven/src/main/java/com/google/cloud/tools/jib/maven/extension/springboot/JibSpringBootExtension.java for implementation.

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