Add emerging subclades to H3N2 HA Nextclade config

Adds emerging subclades as another "clade system" for the Nextclade
workflow. This change allows us to annotate emerging subclades with the
Nextclade dataset produced by this workflow.

nextclade/config/config_dict.yaml

@@ -51,6 +51,9 @@ builds:
         short-clade:
           url: "seasonal_A-H3N2_HA/main/.auto-generated/clades.tsv"
           key: "short-clade"
+        emerging_subclade:
+          url: "seasonal_A-H3N2_HA/emerging/.auto-generated/subclades.tsv"
+          key: "emerging_subclade"
       refs:
         EPI1857216:
           filter: "--min-date 2019 --probabilistic-sampling --group-by year region --min-length 1500 --subsample-max-sequences 2000"