Customization of namespace prefixes

[valentiay]

This PR adds capability to configure namespace prefixes in encoders. Closes #230.

Example

Scala 2

@XmlnsDef("https://example.org/schema", true)
object schema

@XmlCodecNs("foo", schema)
case class Foo(@xmlns(schema) bar: String)

println(XmlEncoder[Foo].encode(Foo("string")))

or

case object schema
implicit val schemaNs: Namespace[schema.type] =
  Namespace.mkInstance(
    uri = "https://example.org/schema",
    preferredPrefix = Some("schema"),
  )

case class Foo(@xmlns(schema) bar: String)
implicit val fooEncoder: XmlEncoder[Foo] = deriveXmlEncoder("foo", schema)

println(XmlEncoder[Foo].encode(Foo("string")))

Output:

<?xml version='1.0' encoding='UTF-8'?>
<schema:foo xmlns:schema="https://example.org/schema">
    <schema:bar>string</schema:bar>
</schema:foo>

Scala 3

case object schema
given Namespace[schema.type] =
  Namespace.mkInstance(
    uri = "https://example.org/schema",
    preferredPrefix = Some("schema"),
  )

case class Foo(@xmlns(schema) bar: String)
given XmlEncoder[Foo] = deriveXmlEncoder("foo", schema)

println(XmlEncoder[Foo].encode(Foo("string")))

Output:

<?xml version='1.0' encoding='UTF-8'?>
<schema:foo xmlns:schema="https://example.org/schema">
    <schema:bar>string</schema:bar>
</schema:foo>

---

Note: If preferred prefix is not specified, encoder will automatically generate prefix. This is still the default behavior.

case object schema
given Namespace[schema.type] =
  Namespace.mkInstance("https://example.org/schema")

case class Foo(@xmlns(schema) bar: String)
given XmlEncoder[Foo] = deriveXmlEncoder("foo", schema)

println(XmlEncoder[Foo].encode(Foo("string")))

<?xml version='1.0' encoding='UTF-8'?>
<ans1:foo xmlns:ans1="https://example.org/schema">
    <ans1:bar>string</ans1:bar>
</ans1:foo>

This document is equal to the documents in examples above. Prefix names don't change the structure of XML document, as far as they are associated with same URIs