Add headers type

[geirolz]

Here a recap of what's inside this PR:

❓Why so many files involved❓

• I've extracted some classes from the model object, that has grown and became too large to handle, into model package, no breaking changes

🔄 Changes:

• Add Headers type, a wrapper of Map[String, AmqpFieldValue] to provide utils
• Add HeaderKey = String type alias.
• Add AmqpFieldDecoder and AmqpFieldEncoder to decode and encode AmqpFieldValue values into scala types.
• Add := syntax to HeaderKey in order to create a tuple of (HeaderKey, AmqpFieldValue) encoding a generic T into a AmqpFieldValue using an implicit AmqpFieldEncoder
• Add .asAmqpFieldValue syntax to any A to encode them into AmqpFieldValue using an implicit AmqpFieldEncoder
• Add .as[T] syntax to AmqpFieldValue to decode it to a generic T using an implicit AmqpFieldDecoder
• Deprecates all type-specific method in EnvelopeDecoder (stringHeader, etc..) in favor of headerAs[IO, T]

💥 Breaking changes:

• AmqpProperties now accept and return an instance of Headers instead of a Map[String, AmqpFieldValue]. The map will be accessible in any case using the .toMap method which exposes the underlying map

Some examples:

import dev.profunktor.fs2rabbit.model.Headers
 
val headers = Headers(
      "x-custom-header"   := "value", // StringVal
      "x-custom-header-2" := 123 // IntVal
)
.updated("x-custom-header-3", 123L) // LongVal
.remove("x-custom-header-3")

/**
* OR
*   val headers = Headers(
*    "x-custom-header" -> StringVal("value"),
*    "x-custom-header-2" -> IntVal(123)
*   )
*   .updated("x-custom-header-3", LongVal(123L))
*   .remove("x-custom-header-3")
* */

headers.get[IO]("x-custom-header") // IO(StringVal("value"))
headers.getOpt("x-custom-header") // Some(StringVal("value"))

headers.getAs[IO, String]("x-custom-header") // IO("value")
headers.getOptAs[String]("x-custom-header") // Some("value")
headers.getOptAsF[IO, Int]("x-custom-header-2") // IO(Some(123))

[matejcerny]

Quite big, not sure if I have time soon for it 😞

[geirolz]

Quite big, not sure if I have time soon for it 😞

Np, let's release now and have a look to this when you have time.
I've changed a lot but I introduced just one braking change, AmqpProperties now wants an instance of Headers instead of Map. I'll try to summarize all changes in the description in order to simplify the review

[geirolz]

@matejcerny I've updated the description. I hope it helps