Update RSA's publicKeyDER method to support exponent any byte length

[WFrost3]

Fixes #1028

The publicKeyDER method for in RSA.swift only supports the conversion of RSA keys that have a exponent that is exactly 3 bytes long.

Most commonly, exponents are either 65537 or 3 and while an exponent of 3 can be vulnerable it is a valid exponent; the common exponents have byte lengths of 3 and 1. In addition, exponents can be non standard and have a byte length that is not 3 or 1.

Checklist:

• Correct file headers (see CONTRIBUTING.md).
• Formatted with SwiftFormat.
• Tests added.

Changes proposed in this pull request:

• publicKeyDER updated to work for exponent of any size
• Test added for 1 byte long exponent

[nathanfallet]

Thank you!