feat: convert string commands to enum

[jmarkerink]

I hope you are appropriating this refactoring with strong typing.

I have modeled the supported/implemented commands in a enumeration and wrapped it in a value object so we can retrieve the original query command string because of the case insensitivity.

Some benefits:

• compile-time safety - typos are caught at compile time
• documentation - enum serves as documentation of all supported commands
• performance - single toLowerCase() with HashMap lookup