Small refactoring to signUrl code.

[houglum]

This commit attempts to refactor some of the functionality in signUrl
such that it's easier to follow and will be easier to add functionality
for generating virtual hosted-style signed URLs. The branch containing
future changes is shown below; this commit is mostly trying to separate
refactoring from new functionality being added:
https://github.com/houglum/google-cloud-java/compare/6e0f6ef..b52205b

This commit also contains a couple of small doc tweaks.

[houglum]

@frankyn FYI, this is to help facilitate adding the signed URL functionality we talked about last week

[codecov]

Codecov Report

Merging #6078 into master will increase coverage by 0.65%.
The diff coverage is 96%.

@@ Coverage Diff @@## master #6078 +/- ##
============================================
+ Coverage 46.73% 47.39% +0.65% - Complexity 25715 27183 +1468 
============================================
Files 2523 2523 Lines 269162 274588 +5426 Branches 30364 31380 +1016 ============================================
+ Hits 125789 130131 +4342 - Misses 133910 134850 +940 - Partials 9463 9607 +144

Impacted Files	Coverage Δ	Complexity Δ
...rc/main/java/com/google/cloud/storage/Storage.java	79.72% <100%> (ø)	0 <0> (ø)	⬇️
...ain/java/com/google/cloud/storage/StorageImpl.java	78.37% <95.65%> (+0.63%)	100 <0> (+2)	⬆️
...re/src/main/java/com/google/cloud/BaseService.java	55.55% <0%> (-6.95%)	2% <0%> (ø)
...re/src/main/java/com/google/cloud/RetryHelper.java	50% <0%> (-3.85%)	2% <0%> (ø)
...le/cloud/compute/deprecated/DeprecationStatus.java	88.63% <0%> (-3.13%)	20% <0%> (ø)
...m/google/cloud/tasks/v2beta3/CloudTasksClient.java	57.43% <0%> (-2.68%)	67% <0%> (+16%)
...va/com/google/cloud/tasks/v2/CloudTasksClient.java	57.43% <0%> (-2.68%)	67% <0%> (+16%)
...m/google/cloud/tasks/v2beta2/CloudTasksClient.java	57.14% <0%> (-2.39%)	79% <0%> (+19%)
...oogle/cloud/firestore/v1/FirestoreAdminClient.java	59.84% <0%> (-2.23%)	38% <0%> (+8%)
.../cloud/scheduler/v1beta1/CloudSchedulerClient.java	55% <0%> (-2.15%)	35% <0%> (+8%)
... and 673 more

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[houglum]

Only presubmit failure was from the linter; I ran mvn com.coveo:fmt-maven-plugin:format and sent a follow-up commit.

[houglum]

@JesseLovelace friendly ping

I've got a couple more PRs queued up after this one (those are more substantial and introduce additional functionality), so reviewing this would unblock me :)