plugin/loadbalance: add limit_answers option

[danhanks]

1. Why is this pull request needed and what does it do?

This PR proposes an addition to the loadbalance plugin enabling it to limit the number of RRs in the Answers section of a DNS response after it has been shuffled / weighted by the plugin.

2. Which issues (if any) are related?

A similar capability is discussed in #6337 with the proposed oneaddr plugin. I commented there about the possibility of expanding that capability from 1 to N. This is the first step in that direction.

3. Which documentation changes (if any) need to be made?

Changes to the README.md (as outlined in this first commit) for the loadbalance plugin, also probably plugin docs on coredns.io.

4. Does this introduce a backward incompatible change or deprecation?

No.

Assuming the syntax / format of this addition are approved, I plan to push additional commits to this PR to implement this proposed change.

[chrisohaver]

syntax looks good

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 58.45%. Comparing base (93c57b6) to head (ab45cf3).
Report is 1135 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6511      +/-   ##
==========================================
+ Coverage   55.70%   58.45%   +2.74%     
==========================================
  Files         224      252      +28     
  Lines       10016    16568    +6552     
==========================================
+ Hits         5579     9684    +4105     
- Misses       3978     6292    +2314     
- Partials      459      592     +133     

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