Add arbitrary string metadata/tags support for snapshot and other API resources

[sjmiller609]

Follow-up from PR #125 review.

Goal

Support optional user-defined string key/value metadata on API resources (starting with snapshots), similar to cloud tags.

Initial scope ideas

• Add metadata: map[string]string where appropriate in OpenAPI/resource types.
• Validate key/value size and character constraints.
• Ensure metadata survives list/get/fork/restore flows.
• Add SDK exposure via Stainless.

Notes

This is intentionally out of scope for PR #125 to keep snapshot lifecycle delivery focused.