Add Rekor OID info. (#1390)

* Add Rekor OID info.

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>

* oid.md: Add tag type.

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

* Remove individual OID fields for single TransparencyLogEntry OID.

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

* oid: Use UTF8STRING for TransparencyLogEntry.

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

---------

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

oid-info.md

@@ -0,0 +1,14 @@
+# Rekor OID Information
+
+## Description
+
+This document defines Rekor
+[OID values](https://github.com/sigstore/sigstore/blob/main/docs/oid-info.md).
+
+Rekor reserves the `1.3.6.1.4.1.57264.3` OID root for all of its values.
+
+## Directory
+
+| OID                   | Name                 | Tag Type     | Description                                                                                                                                                        |
+| --------------------- | -------------------- | ------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
+| 1.3.6.1.4.1.57264.3.1 | TransparencyLogEntry | `UTF8STRING` | Proto serialized [TransparencyLogEntry](https://github.com/sigstore/protobuf-specs/blob/4dbf10bc287d76f1bfa68c05a78f3f5add5f56fe/protos/sigstore_rekor.proto#L89). |