use codegen for `jwt.NumericDate` serialization

Currently we exclude `structs.IdentityClaims` from msgpack code generate (ref [`generate.sh#L11`](https://github.com/hashicorp/nomad/blob/v1.5.0-beta.1/nomad/structs/generate.sh#L11)) because it blows up when the code generator passes over `jwt.NumericDate`, which is a thin wrapper around `time.Time`.

Basically the problem here is that we have `TimeNotBuiltIn` set in our msgpack configuration for backwards compatibility with the original version of the library -- we serialize to the raft logs and snapshots so we're largely stuck with the on-disk structure of any existing serialized types. In theory we could write our own `Ext` that implements the [timestamp extension type](https://github.com/msgpack/msgpack/blob/master/spec.md#timestamp-extension-type) just for `jwt.NumericDate`. That would let us use the fast-path codegen'd encoder from `go-msgpack` rather than the slower reflection-based one.

But this is a bit of a lift and currently only the `ACL.WhoAmI` endpoint passes around `structs.IdentityClaims`. It's never persisted to disk. Taking the slow path on a little used endpoint isn't a big deal, so this is a nice-to-have for some other time.

Start with nomad/structs/generate.sh at the exclusion for structs.IdentityClaims, then trace the msgpack configuration and jwt.NumericDate handling. Review the ACL.WhoAmI path and go-msgpack behavior; done means IdentityClaims uses generated serialization without changing existing raft log or snapshot formats.

Written by the indexing model from the issue text.