HTTP headers in links as dictionary of arrays to support multiple values

[emmanuelmathot]

HTTP 9110 RFC states:

HTTP uses "fields" to provide data in the form of extensible name/value pairs with a registered key namespace. Fields are sent and received within the header and trailer sections of messages ([Section 6](https://www.rfc-editor.org/rfc/rfc9110.html#message.abstraction)).

In order to avoid ambiguity in the field values that are sent, the headers object in a link object should be specified as a dictionary of string arrays Map<string, [string]>

https://stackoverflow.com/questions/3096888/standard-for-adding-multiple-values-of-a-single-http-header-to-a-request-or-resp

[m-mohr]

We should align with STAC spec and aim for the advanced use cases, so I propose:

body to allow any data type
headers to allow Map<string, string | [string]> (an array results in multiple headers)