Rework gcrane options

[jonjohnsonjr]

This allows us to plumb a user-agent everywhere, and makes some things a
bit more consistent. This is getting unwieldy, but it works.

While working on #985 I noticed that we were dropping the useragent in a few places, which is upsetting. This should fix that.

[codecov-commenter]

Codecov Report

Merging #986 (b9b14f5) into main (23990da) will decrease coverage by 0.01%.
The diff coverage is 78.26%.

@@            Coverage Diff             @@
##             main     #986      +/-   ##
==========================================
- Coverage   74.61%   74.60%   -0.02%     
==========================================
  Files         107      107              
  Lines        5027     5040      +13     
==========================================
+ Hits         3751     3760       +9     
- Misses        721      725       +4     
  Partials      555      555              

Impacted Files	Coverage Δ
pkg/v1/google/list.go	62.92% <ø> (ø)
pkg/v1/google/options.go	90.00% <ø> (ø)
pkg/gcrane/options.go	77.77% <66.66%> (-22.23%)	⬇️
pkg/gcrane/copy.go	84.49% <90.90%> (+0.12%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 23990da...b9b14f5. Read the comment docs.