Feature: Add Steam Guard support

[CorySanin]

Addresses #24

Adds Steam Guard as an option. I came from Android where my favorite TOTP apps all had Steam support built-in. Probably missed some things (tests are on their way, unless this PR is a non-starter (created TOTP tests)). Cut me some slack, I've never touched Swift before!

What I've manually tested and verified:

• [x] Correct generation of Steam Guard tokens
• [x] Export/import in 2FAS format v4
• [x] Import from Andotp
• [x] Import from Aegis
• [x] Scanning QR codes from Andotp and Aegis
• [x] Scanning QR codes from KeePassXC
• [x] Manual secret input
• [x] Tokens display correctly in (square) widget
• [x] Unit tests run and pass

if I recall, there are problems with exporting their secret to any other app.

You're right, it's a pain in the neck. I successfully exported mine from my rooted Android phone, but there's a more accessible solution. Create a new secret with steamguard-cli.

[adocyn]

I've checked the task board and we can try to add it to 5.2.5 for testing purposes.

[CorySanin]

Added tests for TOTP token generation