Implement functions for endpoint: Series about fredr HOT 5 CLOSED

I see. I'll go with option A in this PR.

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@DavisVaughan: Just as a heads up, I'm currently working on these functions in a local feature branch.

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Sounds good. I see for the required Params you changed the default from nothing to null. I see the reason you did that (so you can pass to the validation function) but I'd like to ensure the user knows it's required ahead of time. We could either

A) add "required" by the parameter in the documentation

B) restructure the argument checking to be

if(is.missing(search_id)) stop(required_msg, call.=FALSE)
validate_required_param(search_id)

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The function validate_series_id() will throw the error if the user leaves series_id = NULL.

> fredr_series()
 Error: Argument `series_id` must be supplied. 

I don't think there are any endpoints where series_id is an optional parameter.

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I guess im saying, if i was a user and was trying to use this pkg for the first time, how do I know what params are required in the function call the first time I use it? having all of the optional params as param = NULL and the required params as param is a clear way to do that. Or just add it in the docs as "required param" and leave param = NULL for the required params.

Just trying to be uber user friendly and not have the only way they can figure out what is or is not required be running into an error (even if its a helpful one!)

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