Drop optional from common.PublicKey.raw_bytes

[woodruffw]

In practice, the PublicKey message is used in settings that require direct access to the raw_bytes, with the assumption that it isn't missing.

protobuf-specs/protos/sigstore_common.proto

Lines 147 to 155 in 2c9be05

	message PublicKey {
	// DER-encoded public key, encoding method is specified by the
	// key_details attribute.
	optional bytes raw_bytes = 1;
	// Key encoding and signature algorithm to use for this key.
	PublicKeyDetails key_details = 2;
	// Optional validity period for this key, *inclusive* of the endpoints.
	optional TimeRange valid_for = 3;
	}

We should be able to drop the optional qualifier here without causing any real breakage -- proto3 is still implicitly optional everywhere, so dropping optional will just refine the type hints.