Implement ListSessions of BGP API

[sebageek]

VRF filtering and some session stats are still missing here, but listing
sessions does work.

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I'm not 100% on what I should fill in into the SessionStat, that's why I only filled them partly.

[codecov-commenter]

Codecov Report

Merging #271 into master will increase coverage by 0.35%.
The diff coverage is 91.48%.

@@            Coverage Diff             @@
##           master     #271      +/-   ##
==========================================
+ Coverage   59.25%   59.60%   +0.35%     
==========================================
  Files         137      137              
  Lines        7542     7588      +46     
==========================================
+ Hits         4469     4523      +54     
+ Misses       2892     2882      -10     
- Partials      181      183       +2     

Impacted Files	Coverage Δ
protocols/bgp/server/bgp_api.go	78.94% <91.48%> (+25.61%)	⬆️
protocols/bgp/server/metrics_service.go	100.00% <0.00%> (+1.75%)	⬆️
protocols/bgp/server/server.go	25.19% <0.00%> (+7.08%)	⬆️

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[sebageek]

From a quick look it should also be easy to get RoutesImported and RoutesExported from the metrics service. I'll add this to this PR tomorrow.

[sebageek]

I added some stuff. The session API now has a vrf_name field, the description field is filled and vrf_name filtering is implemented.