Add support for generating Steam Guard codes

[bmwalters]

Fixes #211

Overview

• Adds a new parameter to Generator named "Representation". This is an enum with two values: numeric and steamguard.
  • Steam Guard codes are 5 alphanumeric digits instead of base-10.
  • There is only one valid configuration of parameters to obtain a valid Steam Guard token generator: 30 second TOTP, sha1, 5 digits, steamguard representation.
    • To keep the interface more general, this is not enforced in the API contract (such as by adding a separate constructor init(steamGuardSecret:)). Instead, the user of the API is expected to pass the correct parameters.
• Adds test for TOTP values and serialization.
• Updates interface in a hopefully backwards compatible way.
• Users are expected to figure out how to obtain their Steam Guard secret on their own.
• Companion Authenticator PR

[codecov[bot]]

Codecov Report

Merging #225 into develop will decrease coverage by 0.42%. The diff coverage is 96.72%.

@@             Coverage Diff             @@
##           develop     #225      +/-   ##
===========================================
- Coverage    97.38%   96.95%   -0.43%     
===========================================
  Files            6        6              
  Lines          420      427       +7     
===========================================
+ Hits           409      414       +5     
- Misses          11       13       +2     

Impacted Files	Coverage Δ
Sources/Token+URL.swift	96.96% <90.00%> (-1.45%)	:arrow_down:
Sources/Generator.swift	100.00% <100.00%> (ø)
Sources/Keychain.swift	91.89% <0.00%> (-0.68%)	:arrow_down:
Sources/Crypto.swift	100.00% <0.00%> (ø)

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