[Feature] add XML format support

XML is still widely used for configuration in Java/JVM ecosystems. The company I work for uses it to define part of the configurations of such an application. Currently, teams working with XML config files that contain secrets must either convert to a supported format before encrypting with SOPS, or fall back to BINARY mode which loses the structured diff and selective encryption benefits. ## Proposal Add an `xml` store alongside the existing `json`, `yaml`, `ini`, and `dotenv` stores. The implementation would: - Map XML elements to SOPS tree branches, attributes to `@`-prefixed keys, and text content to `#text` keys - Support repeated same-name elements as arrays - Preserve XML comments - Store `sops` metadata as a `<sops>` child element of the root (consistent with how other structured formats handle it) - Reject mixed content (interleaved text and child elements) with a clear error, as this pattern is more common in document markup (HTML, DocBook) rather than in configuration files - Use Go's standard `encoding/xml` for parsing/emission (no new dependencies) I have a working implementation based on the existing store architecture (modeled after the INI store for metadata flattening and the JSON store for hierarchical tree construction). Happy to open a PR if there's interest.