Implement From<String> for Model, Language, and Redact

[jcdyer]

Proposed changes

Add an option to convert strings to various enumerated query options infallibly using From<String>.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update or tests (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING.md doc
• I have added tests and/or examples that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

A customer has requested this feature. It is not the default way to specify the options, because we want to encourage customers to do it in a type-safe manner using the enum where possible. To use this version, one can simply do:

Options::builder()
    .model(String::from("nova-2").into())
    .build()

(Normally, one would probably have a String to begin with, if it were coming from a runtime input value, so the String::from call would not be needed. If it is being created directly in code, the existing enum should be used directly.

Options::builder()
    .model(Model::Nova2)
    .build()

[DamienDeepgram]

LGTM