Support multiple values files

[tiulpin]

This pull request introduces support for handling multiple values files in the Helm chart validation tool. The main changes include the addition of a new ValueFiles type to manage multiple values files, updates to the command-line interface to accept multiple values files, and new tests to verify the merging of these files.

Changes to support multiple values files:

• Added ValueFiles type to manage multiple values files, including methods String and Set to handle command-line input (cmd/main.go).
• Updated the command-line interface to accept multiple values files using the -f flag and modified the usage message accordingly (cmd/main.go).
• Replaced single values file handling with the new ValueFiles type in the main function (cmd/main.go) [1] [2].

Testing enhancements:

• Added a new test TestMergeValues to verify that merging multiple values files produces the expected result, ensuring that values from subsequent files override those from earlier ones (cmd/main_test.go).