Revisit `HttpHeaders` implementing the `MultiValueMap` API

[simonbasle]

See #33823 : the MultiValueMap methods that iterate over the whole header collection misalign with some underlying server implementations.

On the other hand having HttpHeaders implement MultiValueMap brings in some (marginal?) benefits: using CollectionUtils, asserting headers with map-specific assertions, putting all the headers into a plain Map (map.putAll(headers)), etc...

[simonbasle]

Keeping track of notable areas where we check for generic Map for headers (and now should check for both Map and HttpHeaders):

• RequestHeaderMapMethodArgumentResolver
• RequestHeaderMethodArgumentResolver

[simonbasle]

Notable possible additions to HttpHeaders API that could be backported to 6.2.x:

• getOrEmpty using a String key
• getOrDefault (similarly using a String key)
• addAll(HttpHeaders)
• containsHeader(String headerName)
• hasHeaderValues(String, List<String>)
• containsHeaderValue(String, String)
• putAll(HttpHeaders)
• remove(String)
• ...