WIP db provider support for Mysql and Postgres

src/NadekoBot/Db/PostgreSqlContext.cs

@@ -0,0 +1,28 @@
+using Microsoft.EntityFrameworkCore;
+
+namespace NadekoBot.Services.Database;
+
+public sealed class PostgreSqlContext : NadekoContext
+{
+    private readonly string _connStr;
+
+    protected override string CurrencyTransactionOtherIdDefaultValue
+        => "NULL";
+    protected override string DiscordUserLastXpGainDefaultValue
+        => "timezone('utc', now()) - interval '-1 year'";
+    protected override string LastLevelUpDefaultValue
+        => "timezone('utc', now())";
+
+    public PostgreSqlContext(string connStr = "Host=localhost")
+    {
+        _connStr = connStr;
+    }
+
+    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
+    {
+        base.OnConfiguring(optionsBuilder);
+        optionsBuilder
+            .UseLowerCaseNamingConvention()
+            .UseNpgsql(_connStr);
+    }
+}