refactor: update message object to accept array offer param and send offer as string

[perco12]

Description

Ensures that the message object offer input remains an array but output results in message having offer property as a comma-delimited string

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❤️ Thank you!

[wsbrunson]

if this is going to be a comma separated string, should the type just be string?

[perco12]

that is a good call, as the join statement makes the output a generic string, instead of our strict flow type.

[Seavenly]

I think this may be meant to be $ReadOnlyArray<$Values<typeof MESSAGE_OFFER>> | string since we want to accept the input as both and array or comma-delimited string. The merchant input will be ["a", "b"] vs "a,b" for better ergonomics. We convert it internally to a comma-delimited string so that we pass it to the endpoint as a string to get around the zoid dotify encoding issue.

[Seavenly]

Oh apologies. I misspoke and got my wires crossed. ButtonMessage is the output of the function and not the input. 😅 My comment above can be disregarded.