feat: implement ObserveRouters GRPC Service

[margau]

This PR shall allow a streaming update (and removing) of routers with their VRFs, improving the possibility of chaining a ris-mirror behind a RIS while automatically updating the targets.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0e48eb2) 54.34% compared to head (ed69fe7) 52.79%.
Report is 3 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #454      +/-   ##
==========================================
- Coverage   54.34%   52.79%   -1.56%     
==========================================
  Files         146      152       +6     
  Lines        7901     8160     +259     
==========================================
+ Hits         4294     4308      +14     
- Misses       3401     3646     +245     
  Partials      206      206              

Flag	Coverage Δ
unittests	52.79% <ø> (-1.56%)	⬇️

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