internal/grpcutil: Encode timeout headers concisely

[mwilliams-sfo]

30 seconds, for example, should encode as "30S" not "30000000u". Both conform to the specification, but the former uses fewer bits and is more readable in a dump.

RELEASE NOTES: n/a

[linux-foundation-easycla[bot]]

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[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 83.23%. Comparing base (81a00ce) to head (55e45b5). :warning: Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8759      +/-   ##
==========================================
+ Coverage   83.22%   83.23%   +0.01%     
==========================================
  Files         418      418              
  Lines       32385    32386       +1     
==========================================
+ Hits        26952    26958       +6     
+ Misses       4050     4047       -3     
+ Partials     1383     1381       -2     

Files with missing lines	Coverage Δ
internal/grpcutil/encode_duration.go	100.00% <100.00%> (+11.11%)	:arrow_up:

... and 22 files with indirect coverage changes

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