sqldb: add a temporary index to store KV invoice hash to ID mapping

sqldb/sqlc/migrations/000006_invoice_migration.up.sql

@@ -0,0 +1,17 @@
+-- invoice_payment_hashes table contains the hash of the invoices. This table
+-- is used during KV to SQL invoice migration as in our KV representation we
+-- don't have a mapping from hash to add index.
+CREATE TABLE IF NOT EXISTS invoice_payment_hashes (
+     -- id represents is the key of the invoice in the KV store.
+    id INTEGER PRIMARY KEY,
+
+    -- add_index is the KV add index of the invoice.
+    add_index BIGINT NOT NULL,
+
+    -- hash is the payment hash for this invoice.
+    hash BLOB
+);
+
+-- Create an indexes on the add_index and hash columns to speed up lookups.
+CREATE INDEX IF NOT EXISTS invoice_payment_hashes_add_index_idx ON invoice_payment_hashes(add_index);
+CREATE INDEX IF NOT EXISTS invoice_payment_hashes_hash_idx ON invoice_payment_hashes(hash);