Report dandi version in User-Agent header

[jwodder]

This PR sets a User-Agent header when communicating with the new API that contains the version of dandi in use.

[codecov]

Codecov Report

Merging #424 (ece44bd) into master (1cc2043) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #424   +/-   ##
=======================================
  Coverage   82.25%   82.25%           
=======================================
  Files          55       55           
  Lines        5679     5681    +2     
=======================================
+ Hits         4671     4673    +2     
  Misses       1008     1008           

Flag	Coverage Δ
unittests	82.25% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
dandi/dandiapi.py	87.34% <100.00%> (+0.05%)	⬆️
dandi/utils.py	75.83% <100.00%> (+0.08%)	⬆️

---

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 1cc2043...ece44bd. Read the comment docs.

[yarikoptic]

GREAT IDEA! Thank you @jwodder !