allow to merge multiple values/secret files

[olblak]

• fix multiple values file specified
• fix absolute values file path specified

Fix #1403 #1415 #1253

Test

To test this pull request, you can run the following commands:

cd pkg/core/config/
go test

go build -o bin/updatecli .
./bin/updatecli diff --config manifest.yaml --values --values ../tmp/1.yaml --values ./2.yaml --values /tmp/3.yaml

Additional Information

Tradeoff

Potential improvement

[olblak]

Tested the PR on windows and both relative and absolute path worked