no change in behavior, will be used for external references for json
schema

collect_schema overload accepts a list of references directly and does
not save the input schema in the collected_schema list

colleted_schema::get extranct the map of reference_name ->
reference_iobuf
to implement this, a bit of refactoring is applied to the class

…djson

no behavior change in this commit

add a map external_schema_name -> json::Document, change the
mapped_type of schemas_index to an union of local_ptr (json pointer to
the root schema) and external_ptr (ptr to a named external schema in
.external_schemas map)

the type is templated to support rapidjson and jsoncons, this will be
used later

this is done with schema_context::resolve_reference. the function will
get an uri and return a json::Value::ConstObject or throw.

the search is done with schema_index, a local_ptr will be resolved
against the root schema and an external_ptr will resolve against one of
the external_schemas.

note that there is no behavior change yet, because the external_schemas
map is not populated yet

no behavior change

this is done to reduce conversions when parse_json will call itself to
handle external references.

the name change to parse_jsoncons, and a new parse_json calls the former
and applies the translation to rapidjson of the result.

collect_bundled_schema_and_fix_ref has to change too.

no behavior change yet

this commit adds support for external schemas: parse_json gets an
(optional) id for the root schema, and a list of external schemas name
to build into the result.

each external schema il collected, parse_jsoncons invoked on it, and
it's list of bundles schemas is added to the root schemas_index.

each bundled schema has to have an unique id (due to the current
resolve_reference implementation) and the external_schema.$id has to be
unique (if it's not present, the exernal schema name will be used as
$id)

- if we are visiting the root, skip validation as it's already validated
  externally

- if we are visiting a bundled schema, ensure its $id is unique

this commit is a wip an unlikely to be finished.

the idea is to add $ref validation at parse time, and at the same time
handle cleanily various edge cases of external schemas

to do so:
1. external refs are changed to
   https://schema-registry.com/extrnal_ref_name
   this is to make it work like a bundled ref, in resolve_ref
   note: this does not handle well refs/with/slashes in the middle. they
   look like a valid uri path. try to add a test with them and see what
   breaks or not
2. at collect_bundled_schemas_and_fix_ref, first do a tree traversal for
   id, and save refs in an output parameter unresolved_ref.
   after the first run, check every ref: if it's not a bundled/local
   ref, or an external ref, throw. bundled/local ref are made absolute,
   external refs are encoded as (1)
   a new parameter controls what to do if a ref is unknown. for the root
   schema, an unknown ref should raise an error. for an external schema,
   assume that it's external, because parse_jsocons is recursive
   and gets invoked for each external schema to extract
   their bundled schemas index.
   it is assumed that external schemas are already good (no unresolved
   refs) because they are already accepted, so we can limit the
   recursion to 2 levels (root, and then one for each direct/indirect
   external schema)
3. schema_context::resolve_reference does not need to handle external
   schemas anymore, since they appear like bundled schemas (the uri is
   either in the index or not). that code can be simplified