[JENKINS-66326] Prepare SCM Sync Configuration for core Guava upgrade

[basil]

Downstream of #70. See JENKINS-66326 and JENKINS-65988. Jenkins core is using Guava 11.0.1, which was released on January 9, 2012. Jenkins core would like to upgrade to Guava 30.1.1, which was released on March 19, 2021. Plugins must be prepared to be compatible with both Guava 11.0.1 and Guava 30.1.1 in advance of this core transition.

In particular, this plugin has been identified as using the following methods which have been removed in the latest version of Guava:

• com/google/common/io/ByteStreams#getChecksum
• com/google/common/io/ByteStreams#newInputStreamSupplier
• com/google/common/io/Files#getChecksum

These methods existed in Guava 11.0.1 but have been removed in recent versions.

To facilitate the Jenkins core transition, this plugin must be prepared and released such that it works with both Guava 11.0.1 and latest. This PR rewrites the relevant code to avoid the use of Guava and use native Java Platform functionality instead.

CC @guipal @rodrigc

[basil]

@guipal @rodrigc Would it be possible to take a look at this PR?

[basil]

@guipal @rodrigc Gentle ping. It would be nice to get this PR merged and released to facilitate the upcoming Guava upgrade in Jenkins core (JEP-233).