Add Shodan API plugin for DNS subdomain enumeration

[kato83]

Sorry, I closed #1118 because it included another implementation I was working on at the same time.

Re-add Shodan DNS plugin for v5.1.1

Summary

Re-add the Shodan subdomain enumeration plugin, which was removed upstream in v5, ported to the current API.

Changes

engine/plugins/api/shodan.go (new file)

Shodan plugin rewritten for the v5.1.1 API:

• Handler.Priority → Handler.Position + MaxInstances
• http.RequestWebPage(ctx, req) → amasshttp.RequestWebPage(ctx, client, req)
• Use e.Session.Ctx(), e.Session.NetSem(), and e.Session.Clients().General
• Remove lookup() method (depends on removed SourceToAssetsWithinTTL)
• Update import aliases (dns → amassdns, http → amasshttp)

engine/plugins/load.go

Register api.NewShodan in the plugin load list.