Adds codecov yml file to reduce flakiness in coverage check

[dbbaughe]

Issue #, if available:

Description of changes:
Adds the codecov yml config and sets the minimum target coverage to 70% while allowing a 1% drop.
This should hopefully help with the random X because of test randomization. This only makes the change for the project status check, will see if we need something similar for patch after a few more PRs.

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 #251 into master will decrease coverage by 0.10%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #251      +/-   ##
============================================
- Coverage     71.48%   71.37%   -0.11%     
  Complexity      633      633              
============================================
  Files            86       86              
  Lines          3647     3647              
  Branches        601      601              
============================================
- Hits           2607     2603       -4     
- Misses          729      730       +1     
- Partials        311      314       +3     

Impacted Files	Coverage Δ	Complexity Δ
...ch/indexstatemanagement/model/destination/Chime.kt	14.28% <0.00%> (-42.86%)	3.00% <0.00%> (-1.00%)
...ticsearch/indexstatemanagement/model/Transition.kt	72.54% <0.00%> (-7.85%)	5.00% <0.00%> (ø%)
...exstatemanagement/model/destination/Destination.kt	47.05% <0.00%> (ø)	6.00% <0.00%> (ø%)
...ch/indexstatemanagement/model/destination/Slack.kt	57.14% <0.00%> (+42.85%)	4.00% <0.00%> (+1.00%)

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