Add Codecov configuration to set a coverage threshold to pass the check on a commit

[ftianli-amzn]

Issue #, if available:

Description of changes:

Add codecov.yml configuration file and set the minimum target coverage to 70% while allowing a 1% drop.
Without those config, the check on a commit will not pass.
Reference: opendistro-for-elasticsearch/index-management#251

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Merging #231 into master will increase coverage by 0.11%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #231      +/-   ##
============================================
+ Coverage     76.14%   76.25%   +0.11%     
  Complexity      157      157              
============================================
  Files            63       63              
  Lines          2586     2586              
  Branches        402      402              
============================================
+ Hits           1969     1972       +3     
+ Misses          421      418       -3     
  Partials        196      196              

Impacted Files	Coverage Δ	Complexity Δ
...distroforelasticsearch/alerting/core/JobSweeper.kt	71.89% <0.00%> (+0.54%)	35.00% <0.00%> (ø%)
...roforelasticsearch/alerting/alerts/AlertIndices.kt	67.46% <0.00%> (+1.58%)	0.00% <0.00%> (ø%)

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