make Config.Load concurrent-safe

[ktong]

So Load and Watch can be called on different goroutines.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.0%. Comparing base (a2af1b4) to head (cdb9243).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #567   +/-   ##
=====================================
  Coverage   82.9%   83.0%           
=====================================
  Files         62      62           
  Lines       3294    3303    +9     
=====================================
+ Hits        2733    2742    +9     
  Misses       478     478           
  Partials      83      83           

Files with missing lines	Coverage Δ
config.go	100.0% <100.0%> (ø)
default.go	100.0% <ø> (ø)
provider.go	100.0% <100.0%> (ø)
watch.go	100.0% <100.0%> (ø)

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