fix(types): support metadata override in file.copy()

[stephenplusplus]

Fixes #1398

file.copy() allows overwriting the original file's metadata:

const originalFile = bucket.file('original-file-name')
const [copiedFile] = await originalFile.copy('new-file-name', {metadata: {thisIsACopy: 'true'}})
const [metadata] = await copiedFile.getMetadata()
// metadata.metadata.thisIsACopy === 'true'

But, we didn't document it through JSDocs or type docs. This fixes that and adds a system test.

[codecov]

Codecov Report

Merging #1406 (8c97684) into master (65f3ee5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1406   +/-   ##
=======================================
  Coverage   99.10%   99.10%           
=======================================
  Files          14       14           
  Lines       12143    12145    +2     
  Branches      529      602   +73     
=======================================
+ Hits        12034    12036    +2     
  Misses        109      109           

Impacted Files	Coverage Δ
src/file.ts	99.94% <100.00%> (+<0.01%)	⬆️

---

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 65f3ee5...f1bd592. Read the comment docs.