Added support for a wider variety of primitive types for the Credentials expiration date

[seanmcneil]

Changes

Please describe both what is changing and why this is important. Include:

Modifies the approach used to set the expiration date on Credentials such that it can handle a wider variety of primitive types.

References

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#439

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Testing

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• This change adds unit test coverage
• This change has been tested on the latest version of the platform/language or why not

Checklist

• I have read the Auth0 general contribution guidelines
• All existing and new tests complete without errors
• All active GitHub checks have passed

[Widcket]

@seanmcneil thanks for this PR. Since numeric types conform to LosslessStringConvertible, String(describing:) is safe to use for this.