Add SubjectPublicKeyInfo DER newtype

[lvkv]

This PR adds a new pub struct SubjectPublicKeyInfo<'a>(Der<'a>); type that represents an X.509 certificate's SubjectPublicKeyInfo (also known as an SPKI) as specified in RFC 5280.

I've also added a few impls that mirror what exists for other Der newtypes, like CertificateDer:

• AsRef<[u8]>
• Deref
• From<&'a [u8]>
• From<Vec<u8>>
• pub fn into_owned(self)

The motivation behind this is to make it possible to verify consistency for public and private keys (rustls/rustls#1918) —which can be accomplished by comparing SPKI values. CC @cpu btw.

[cpu]

Looks great. Thanks!

Would you like to also add a second commit that updates Cargo.toml's version from 1.5.0 to 1.6.0?

[lvkv]

Would you like to also add a second commit that updates Cargo.toml's version from 1.5.0 to 1.6.0?

Done!

[djc]

Great, thanks!

[djc]

• Published rustls-pki-types v1.6.0 at registry crates-io
• [new tag] v/1.6.0 -> v/1.6.0
• Published release notes at https://github.com/rustls/pki-types/releases/tag/v%2F1.6.0