Add ENV variable handler to provide custom configuration to dbsync

This commit introduces a new environment variable,
CARDANO_DB_SYNC_CONFIG, allowing users to specify a custom configuration
file for the Cardano db-sync service. The modification in
`nix/nixos/cardano-db-sync-service.nix` adds a conditional check to use
the custom configuration file if the environment variable is set,
providing more flexibility in deployment scenarios and facilitating
custom configurations without altering the base code. This change
addresses a need for dynamic configuration management as specified in
the recent user requirements.

Author: placek

nix/nixos/cardano-db-sync-service.nix

@@ -217,13 +217,23 @@ in {
               LEDGER_OPTS="--state-dir ${cfg.stateDir}"
             fi
 
-            exec ${exec} \
-                --config ${configFile} \
-                --socket-path "$CARDANO_NODE_SOCKET_PATH" \
-                --schema-dir ${self.schema} \
-                ${toString cfg.rtsArgs} \
-                ''${LEDGER_OPTS} \
-                ''${EXTRA_DB_SYNC_ARGS:-}
+            if [[ -z "''${CARDANO_DB_SYNC_CONFIG:-}" ]]; then
+              exec ${exec} \
+                  --config ${configFile} \
+                  --socket-path "$CARDANO_NODE_SOCKET_PATH" \
+                  --schema-dir ${self.schema} \
+                  ${toString cfg.rtsArgs} \
+                  ''${LEDGER_OPTS} \
+                  ''${EXTRA_DB_SYNC_ARGS:-}
+            else
+              exec ${exec} \
+                  --config "$CARDANO_DB_SYNC_CONFIG" \
+                  --socket-path "$CARDANO_NODE_SOCKET_PATH" \
+                  --schema-dir ${self.schema} \
+                  ${toString cfg.rtsArgs} \
+                  ''${LEDGER_OPTS} \
+                  ''${EXTRA_DB_SYNC_ARGS:-}
+            fi
           '';
     in {
       pgpass = builtins.toFile "pgpass" "${cfg.postgres.socketdir}:${