Format change from .db to .bustub

[prashanthduvvada]

Changed from ".db" to ".bustub" as per given requests.

[connortsui20]

lgtm!

Also, are we are not changing the names in the tools folder?

│❯ rg "\.db\"" ./tools                                                                                                                                                                         │
│./tools/wasm-bpt-printer/wasm-bpt-printer.cpp                                                                                                                                                 │
│60:  auto *disk_manager = new DiskManager("test.db");                                                                                                                                         │
│                                                                                                                                                                                              │
│./tools/b_plus_tree_printer/b_plus_tree_printer.cpp                                                                                                                                           │
│75:  auto *disk_manager = new DiskManager("test.db");                                                                                                                                         │
│139:  remove("test.db");                                                                                                                                                                      │
│                                                                                                                                                                                              │
│./tools/shell/shell.cpp                                                                                                                                                                       │
│32:  auto bustub = std::make_unique<bustub::BustubInstance>("test.db");                                                                                                                       │
│                                                                                                                                                                                              │
│./tools/sqllogictest/sqllogictest.cpp                                                                                                                                                         │
│260:    bustub = std::make_unique<bustub::BustubInstance>("test.db");