Feature request: Move transformation logic to gIobal scope

[aslafy-z]

I've noticed that several providers support transformations such as:

• Base64 encoding for file provider
• Trimming newlines for GCPSecrets provider

It would be very useful if this transformation logic were moved to a global scope, making it accessible across all providers. This would improve flexibility and ensure that users can apply common transformations regardless of the provider being used.

I could think of some additional transformers:

• Base64 decoding (#199)
• Case modification
• Url encoding
• htpasswd format encoding
• Sops decoding (https://github.com/helmfile/vals/issues/35)

[yxxhero]

@aslafy-z PR is welcome.