feat: add setLogicalTimeout helper to RetrySettings

[noahdietz]

Adds a helper to the RetrySettings.Builder to set a logical call timeout. setLogicalTimeout takes the given timeout and applies it to each of the three timeout settings in RetrySettings and sets the timeout multiplier to one to keep it constant. In combination with the ExponentialRetryAlgorithm, the first RPC attempt will have the entire timeout, and any subsequent attempts will have only the time remaining in the timeout. Effectively, users will see their callable "timeout" and return an error after their given timeout has been used up. Under the hood, each RPC attempt respects the deadline calculated from the start of the first attempt with the given timeout, and will use as much time as is left.

[codecov]

Codecov Report

Merging #1334 (6d80e1b) into master (49efdc3) will increase coverage by 0.18%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1334      +/-   ##
============================================
+ Coverage     80.34%   80.53%   +0.18%     
  Complexity     1331     1331              
============================================
  Files           210      210              
  Lines          5682     5686       +4     
  Branches        519      519              
============================================
+ Hits           4565     4579      +14     
+ Misses          912      895      -17     
- Partials        205      212       +7     

Impacted Files	Coverage Δ	Complexity Δ
...ava/com/google/api/gax/retrying/RetrySettings.java	55.76% <100.00%> (+24.51%)	2.00 <0.00> (ø)

---

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 49efdc3...6d80e1b. Read the comment docs.