Add public method for key.requires_touch()

[timweri]

This pull request introduces a new TouchRequirement API to standardize how the codebase determines whether a hardware-backed key requires user touch for signing operations. The changes add the TouchRequirement enum, implement it for SSH and YubiKey keys, and expose it through public APIs.

[Copilot]

Pull request overview

This PR adds a public requires_touch() API to expose “user presence required” / touch requirement information for hardware-backed SSH keys (SK/FIDO) and YubiKey PIV slots, and expands test coverage around the new behavior.

Changes:

• Add requires_touch() on EcdsaSkPrivateKey, Ed25519SkPrivateKey, and PrivateKey (plus a convenience wrapper on FIDOSSHKey).
• Add YubiKey PIV helpers to fetch touch policy metadata and compute whether touch is required.
• Add/extend tests validating requires_touch() for SK keys and non-SK keys.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 6 comments.

File	Description
tests/sk-privkey.rs	Adds assertions and new tests for PrivateKey::requires_touch() behavior across SK/non-SK keys and flag values.
src/yubikey/piv/management.rs	Introduces touch policy -> touch-required mapping plus new fetch_touch_policy() / requires_touch() APIs and unit tests.
src/ssh/privkey.rs	Implements requires_touch() for SK private key types and on the top-level PrivateKey.
src/fido/generate/mod.rs	Exposes requires_touch() on generated FIDOSSHKey by delegating to the underlying PrivateKey.

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