Events: lift `value` type restriction to allow any DAG-CBOR encoded type

[raulk]

Event entry values are defined to be DAG-CBOR in the FIP. This enables introspection through a self-describing format. However, we need to decide if this is just a recommendation, or we effectively enforce it in the FVM by validating with as little overhead as possible (we don't care about the payload, and we don't care to traverse any links in it, we just care that it's correct DAG-CBOR).

Long (heated) discussion here: #1049 (comment)

Next step is to take it to a broader audience within the FIP discussion, and to come back here and implement the outcome.

[raulk]

We have simplified this problem for M2.1. FIP-0049 now specifies the type of this field as any DAG-CBOR, but subsequently restricts it to only CBOR Major Type 2 (byte strings). So we can implement the value field as Vec<u8> today and defer this decision to a later milestone when we lift this restriction.

[Stebalien]

Current status: We've now specified the value field as "an IPLD block" and have added a codec field. We currently restrict this to "raw", but will likely allow other codecs in the future.