allow configuring gRPC default service config for trillian client

[bobcallaway]

this allows deployers to configure trillian client load balancing & timeouts

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 25.00%. Comparing base (488eb97) to head (43c22da).
⚠️ Report is 459 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2549       +/-   ##
===========================================
- Coverage   66.46%   25.00%   -41.46%     
===========================================
  Files          92      189       +97     
  Lines        9258    24415    +15157     
===========================================
- Hits         6153     6105       -48     
- Misses       2359    17545    +15186     
- Partials      746      765       +19     

Flag	Coverage Δ
e2etests	46.91% <100.00%> (-0.65%)	⬇️
unittests	16.20% <0.00%> (-31.48%)	⬇️

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