Add optional features for DES/3DES

[grovesNL]

Fixes #68

---

Note

Adds optional features for DES/3DES key decryption, enables docs.rs to build with all features, and documents usage/features in the README.

• Config (Cargo.toml):
  • Add crate features decrypt-des (pkcs8/des-insecure) and decrypt-3des (pkcs8/3des) for legacy key decryption.
  • Enable docs.rs builds with all features via package.metadata.docs.rs.all-features = true.
• Docs (README.md):
  • Add Usage section with example.
  • Document optional features: decrypt-des and decrypt-3des.

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[kenkoooo]

Thanks! Overall this looks good. One naming question: would the decrypt-* prefix clearly convey that these flags enable specific pkcs8 algorithms? In the upstream pkcs8 crate the features are named after the algorithms (e.g., 3des, des-insecure) and organized under pkcs5/encryption as needed, so names like pkcs8-3des and pkcs8-des-insecure might be clearer to users who are familiar with PKCS#8 and want to enable those features. Do we have any prior art using a decrypt-* prefix for PKCS#8 feature flags?

[grovesNL]

Sure, we could use pkcs8-3des and pkcs8-des-insecure as the names if you'd like. I'm not sure aware of any prior art.

I was treating pkcs8 as an internal implementation detail, so the features were meant to enable decryption of DES and 3DES (standard names known outside of the pkcs8 crate).