Make Aura-specific dependency optional

[nazar-pc]

Motivation

We use frontier in a project with custom consensus and s[c|p]-consensus-aura dependencies are just extra things for us to build that we will not use.

Suggested Solution

Hide Aura-specific dependencies (sp-consensus-aura, sc-consensus-aura doesn't seem to be used at all and should be removed) in fc-rpc behind optional (even if enabled by default) feature, so it can be disabled.

Alternatives

Alternative would be to remove it from RPC completely since anyone should be able to implement AuraConsensusDataProvider in their codebase, but I assume there was a reason for it to exist, so I didn't suggest removing it outright.