[SPARK-3996]: Shade Jetty in Spark deliverables

[pwendell]

(v2 of this patch with a fix that was only relevant for the maven build).

This patch piggy-back's on @vanzin's work to simplify the Guava shading,
and adds Jetty as a shaded library in Spark. Other than adding Jetty,
it consilidates the 's into the root pom. I found it was
a bit easier to follow that way, since you don't need to look into
child pom's to find out specific artifact sets included in shading.

[SparkQA]

Test build #26367 has started for PR 4285 at commit d3e7f4e.

• This patch merges cleanly.

[vanzin]

LGTM. Just looked at the last commit, though, assumed the others are the same.

[SparkQA]

Test build #26367 has finished for PR 4285 at commit d3e7f4e.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test FAILed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/26367/
Test FAILed.

[pwendell]

Jenkins, test this please.

[SparkQA]

Test build #26415 has started for PR 4285 at commit d3e7f4e.

• This patch merges cleanly.

[SparkQA]

Test build #26415 has finished for PR 4285 at commit d3e7f4e.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/26415/
Test PASSed.

[pwendell]

Okay - let's try this for take 2.