Simplify NetworkIdentity type

[eddyashton]

Following #6660, this PR simplifies the NetworkIdentity type by flattening it to a single type. The core of the changes are those in identity.h, which removes ReplicatedNetworkIdentity and several fields from NetworkIdentity. This touches a bunch of other files for 2 reasons:

• Moved COSESignaturesConfig under the ccf:: namespace, since it is in a public header.
• Rather than accessing cose_signatures_config from NetworkIdentity, the /join endpoint in node_frontend.h now accesses the single instance stored in/owned by history.h, which takes a few hops to reach. I think some of these hops are helpful, some may be unnecessary, but regardless this fits the existing style.

While we're doing this, be slightly stricter about the order of calls to history, and ensure we throw errors if set_service_signing_identity is called multiple times, or if set_service_signing_identity was not called before either get_cose_signatures_config or emit_signature.

[achamayou]

This was deliberately not namespaced, because it started under startup_config.h, out of shallow consistency with other config types. I do not remember why we don't namespace those, and the right thing to do is perhaps to namespace them all.

[achamayou]

ServiceSigningIdentity seems slightly more precise.