GroupVerify 0.3.0: check players with the Steam Web API as they join, clan tag for other plugins

Steam rate-limits its community member lists by IP (429), so the whole-list download is gone.
Each player is checked as they join with ISteamUser/GetUserGroupList: a member's 'group' row is
added or refreshed, a non-member's deleted, and when Steam won't say (private profile, bad key,
outage) the database stands. A player is asked about at most once per
min_forced_refresh_seconds; css_verify rechecks its targets, offline SteamIDs too. Changing the
group clears the old group's rows. refresh_minutes is gone.

The Web API key goes in configs/plugins/GroupVerify/GroupVerify.json (SteamWebApiKey); every
other setting stays in the database.

New clan_tag setting and css_gv_clantag <tag|-> (@css/root): the scoreboard tag for verified
players without a rank, applied by CS2-Tags. The API gains ClanTag and OnClanTagChanged, so
shared/GroupVerifyApi changes: needs a server restart.

Config.cs

@@ -0,0 +1,15 @@
+using System.Text.Json.Serialization;
+using CounterStrikeSharp.API.Core;
+
+namespace GroupVerify;
+
+// configs/plugins/GroupVerify/GroupVerify.json, which CSS writes with these defaults on first load.
+// Only the Steam Web API key lives here, since it's a secret; every other setting is in the
+// database (gv_settings). Changes need the plugin reloaded: css_plugins reload GroupVerify.
+public class GroupVerifyConfig : BasePluginConfig
+{
+    // From https://steamcommunity.com/dev/apikey. Without it, nobody's group membership is
+    // checked, and only what's already in the database counts.
+    [JsonPropertyName("SteamWebApiKey")]
+    public string SteamWebApiKey { get; set; } = "";
+}