Allow optional fields to be explicitly set to undefined

[haines]

This PR allows fields to be set to undefined explicitly even when the TypeScript compiler option exactOptionalPropertyTypes is enabled.

Currently optional message fields are typed as

field?: Type

but with exactOptionalPropertyTypes this means you can't set them to undefined; you have to omit them. This is a bit inconvenient if you're trying to pass through a value from a variable.

This PR changes the optional fields to be typed as

field?: Type | undefined

which removes the restriction.

[smaye81]

Hey @haines thank you for the PR. Appreciate you being patient.

Our approach has been that we support the default compiler settings for a particular version of TypeScript. And since exactOptionalPropertyTypes: true is not a default setting, I don't think it's something we want to factor into the generated code. It's just a can of worms we are trying to avoid because trying to handle the myriad options in tsconfig would be a difficult task.

That being said, if this becomes a default setting, we would certainly support it at that time. Thanks again for the consideration.