feat: implemented watchdog as BackgroundResource

[rahulKQL]

This change updates watchdog to BackgroundResource, Also, watchdog unschedules itself explicitly when shutdown.

closes #828

[codecov]

Codecov Report

Merging #838 into master will decrease coverage by 0.11%.
The diff coverage is 84.61%.

@@             Coverage Diff              @@
##             master     #838      +/-   ##
============================================
- Coverage     78.73%   78.61%   -0.12%     
- Complexity     1151     1161      +10     
============================================
  Files           202      202              
  Lines          5107     5140      +33     
  Branches        407      413       +6     
============================================
+ Hits           4021     4041      +20     
- Misses          913      925      +12     
- Partials        173      174       +1

Impacted Files	Coverage Δ	Complexity Δ
...gle/api/gax/rpc/InstantiatingWatchdogProvider.java	90.9% <100%> (-0.76%)	16 <1> (+1)
.../com/google/api/gax/rpc/FixedWatchdogProvider.java	100% <100%> (ø)	10 <1> (+1)	⬆️
...ain/java/com/google/api/gax/rpc/ClientContext.java	80.88% <50%> (-0.94%)	9 <0> (+1)
...src/main/java/com/google/api/gax/rpc/Watchdog.java	80.5% <85.71%> (+0.91%)	11 <5> (+5)	⬆️
...main/java/com/google/api/gax/grpc/ChannelPool.java	45.9% <0%> (-3.04%)	13% <0%> (+2%)
...api/gax/grpc/InstantiatingGrpcChannelProvider.java	79.89% <0%> (-0.22%)	35% <0%> (ø)

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[igorbernstein2]

LGTM!
Thanks for fixing this