Add agent endpoint for resolving tags

[codygibb]

Allows agent to proxy get tag requests to build-index.

Useful for non-docker client which want to use build-index to manage tags.

[codecov-io]

Codecov Report

Merging #149 into master will increase coverage by 0.33%.
The diff coverage is 73.07%.

@@            Coverage Diff             @@
##           master     #149      +/-   ##
==========================================
+ Coverage   67.21%   67.55%   +0.33%     
==========================================
  Files         178      178              
  Lines        8654     8678      +24     
==========================================
+ Hits         5817     5862      +45     
+ Misses       2097     2078      -19     
+ Partials      740      738       -2

Impacted Files	Coverage Δ
agent/agentserver/client.go	84% <66.66%> (-16%)	⬇️
agent/agentserver/server.go	68.6% <78.57%> (+1.03%)	⬆️
lib/torrent/scheduler/conn/conn.go	55.22% <0%> (+1.49%)	⬆️
origin/blobserver/server.go	70.71% <0%> (+1.9%)	⬆️
lib/torrent/scheduler/events.go	72.36% <0%> (+4.02%)	⬆️
lib/torrent/scheduler/scheduler.go	75% <0%> (+4.08%)	⬆️
utils/dedup/interval_trap.go	100% <0%> (+10%)	⬆️

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