resolver: create AddressMapV2 with generics to replace AddressMap

[dfawley]

RELEASE NOTES:

• resolver: Add experimental AddressMapV2 with generics to ultimately replace AddressMap. Deprecate AddressMap for deletion.

[codecov]

Codecov Report

Attention: Patch coverage is 94.59459% with 2 lines in your changes missing coverage. Please review.

Project coverage is 81.97%. Comparing base (d8924ac) to head (e3ea5e5).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
balancer/pickfirst/pickfirstleaf/pickfirstleaf.go	94.11%	1 Missing ⚠️
resolver/map.go	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8187      +/-   ##
==========================================
- Coverage   81.99%   81.97%   -0.03%     
==========================================
  Files         410      410              
  Lines       40237    40233       -4     
==========================================
- Hits        32993    32981      -12     
- Misses       5873     5884      +11     
+ Partials     1371     1368       -3     

Files with missing lines	Coverage Δ
balancer/base/balancer.go	48.29% <100.00%> (-0.70%)	⬇️
balancer/weightedroundrobin/balancer.go	83.42% <100.00%> (ø)
balancer/pickfirst/pickfirstleaf/pickfirstleaf.go	86.60% <94.11%> (-0.11%)	⬇️
resolver/map.go	94.39% <92.30%> (-1.80%)	⬇️

... and 28 files with indirect coverage changes

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[arjan-bal]

I don't have any reason to believe this is used widely (if at all) outside our project, and it's experimental, and it's very easy to migrate to, so I believe it's fine to just swap in place without a migration period. Let me know if you feel strongly otherwise.

RELEASE NOTES:

• resolver: convert experimental AddressMap to use generics

Discussed offline and we found usages in SourceGraph. The PR now keeps the old AddressMap and deprecates it.