[2.0] Helidon 4.x change - Http.Headers, Http.Header renaming

[rob-bygrave]

Helidon is making the following changes:

• Http.HeaderNames - known header names and factory methods for HeaderName (used to be Header)
• Http.Header - header name and value (used to be HeaderValue)
• Http.Headers - known headers and factory methods for Header (used to be Header and HeaderValues)
• Additional factory and instance methods now exist to align signatures (methods with String as header name, and methods with int and long as header value)
• Known HttpMediaTypes are now in HttpMediaTypes, values without parameters should be used from MediaTypes