support folding headers according to RFC5322 for outlook add in

https://datatracker.ietf.org/doc/html/rfc5322 (https://datatracker.ietf.org/doc/html/rfc5322#section-2.2.3) supports header folding so headers with more than 998 characters can be used in email.

unfortunately office-js doesn't support that and has a fixed maximum length of 998 which it seems to check with a simple check for string length.

If this check is removed, office-js doesn't complain but outlook itself (or the API of it) complains with either a formatting error 2006 (outlook 2016) or without any error (MS 365 with on prem exchange and outlook.office365.com)

Here is an example header

my-type="urn:my:test:which:is:longer:than:998:1:0";
 my-param*0="PG1iOadpbmRpbmdJbmZvcm1hdGlvbiB4bWxuczp4c2k9Imh0dH";
 my-param*1="A6Ly93d3cudzMub3JnLzIwMDEvWE1MU2NoZW1hLWluc3RhbmNl";
 my-param*2="IiB4bWxdfgg4c2Q9Imh0dHA6Ly93d3cudzMub3JnLzIwMDEvWE";
 my-param*3="1MU2NoZW1hIiB4bWxuczpkcz0iaHR0cDovL3d3dy53My5vcmcv";
 my-param*4="MjAwMC8wOS94bWxkc2lnIyIgeG1sbnM6eG1pbWU9Imh0dHA6Ly";
 my-param*5="93d3cudzMub3JnLzIwMDUvMDUveG1sbWltZSIgeG1sbnM6bWI9";
 my-param*6="InVybjpuYXR934dfgfdhZzo0Nzc4OmJpbmRpbmdpbmZvcm1hdG";
 my-param*7="lvbjoxOjAiPjxtYjpNZXRhZGF0YUJpbmRpbmdDb250YWluZXI+";
 my-param*8="PG1iOk1ldGFkYXRhQmluZGluZz48bWI6TWV0YWRhdGE+PHNsYW";
 my-param*9="I6b3JpZ2luYXRvckNvbmZpZGVudGlhbGl0eUxhYmVsIHhtbG5z";
 my-param*10="OnNsYWI9InVybjpuYXRvOnN0YW5hZzo0Nzc0OmNvbmZpZGVudG";
 my-param*11="lhbGl0eW1ldGdfgfdgfdagfdaggTowIj48c2xhYjpDb25maWRl";
 my-param*12="lhbGl0eW1ldGdfgfdgfdagfdaggTowIj48c2xhYjpDb25maWRl";
 my-param*13="lhbGl0eW1ldGdfgfdgfdagfdaggTowIj48c2xhYjpDb25maWRl";
 my-param*14="lhbGl0eW1ldGdfgfdgfdagfdaggTowIj48c2xhYjpDb25maWRl";
 my-param*15="lh1sdffW1ldGFkYXRhbGFiZWw6MTowIj48c2xhYjpDb25maWRl";

this header is valid according to RFC 5322 and I try to add it like

export async function addHeader(data) {
 let result = false
 const item = Office.context.mailbox.item
 await item.internetHeaders.setAsync({
        "my-header": data
  }, function (asyncResult) {
        console.log('got result')
        console.log(asyncResult.status)
        result  = asyncResult.status === Office.AsyncResultStatus.Suceeded
   })
}

My current use case is to use it for labels like described inhttps://www.rfc-editor.org/rfc/rfc7444.html but there are surely other use cases as well.
Implementing this check wouldn't be very complicated, we did it ourselves for our product.

as side information:
VSTO plugins can succesfully add this header to the email.

Please add support for RFC 5322 folding headers.

here is the link to the github issues -> https://github.com/OfficeDev/office-js/issues/3546