This defines OIDs for most log entry fields (though not all). This is
primarily intended to be used by clients that want to represent Rekor
data in different formats and/or cannot use bundles.

For example, gitsign cannot use the verification bundle since it uses
PKCS7 signatures to keep compatibility with existing Git clients /
signature formats. Since PKCS7 already defines a structure for payload
hashes, it implements offline verification by deconstructing bundles
into distinct OIDs then reconstructs them for verification.

Signed-off-by: Billy Lynch <billy@chainguard.dev>

Signed-off-by: Billy Lynch <billy@chainguard.dev>

Signed-off-by: Billy Lynch <billy@chainguard.dev>

Signed-off-by: Billy Lynch <billy@chainguard.dev>