Allow for .yaml extension of appspec

According to this one:

https://stackoverflow.com/questions/21059124/is-it-yaml-or-yml/21059164

.yaml is the preferred extension to .yml

I think it does make sense that appspec could be appspec.yml AND appspec.yaml. A current codeDeploy task with appspec.yaml instead of appspec.yml fails with:

`The CodeDeploy agent did not find an AppSpec file within the unpacked revision directory at revision-relative path "appspec.yml". The revision was unpacked to directory
....
...
...`

The proper approach should be to allow for the deprecated .yml format but also to allow for .yaml file as that's the recommended extension.

Start by reviewing the CodeDeploy sample entry points and the AppSpec filenames mentioned in the issue. Check where the required appspec.yml name is documented or enforced, then verify that both appspec.yml and appspec.yaml are accepted while the existing .yml format continues to work.

Written by the indexing model from the issue text.