[copybara-service]

Composable prompting protocol names: base[:version](+feature[:version])*.

Precursor for protocol features such as python+verbatim, which is the next
change. This one registers no feature, so existing names (python,
python:1.0, python:2.0, json, markdown, ...) prompt, parse and are
recorded exactly as before.

WHAT

• One parser, ProtocolName.parse (schema/base.py), reads the grammar and
  normalizes a name: a feature without a version takes its only registered
  version, implied features are added, and features are put in registration
  order. Once the base version is filled in, a protocol has exactly one name.
• Names are checked where they are used: a malformed name, an unknown or
  repeated feature, an unknown feature version, or a feature that does not
  support the base protocol or its version raises ValueError.
• lf.query records the normalized name (python -> python:2.0, as before)
  and now also passes it to the LfQuery instance, which used to keep its
  class default 'python', so prompt and parse see the same name as the
  record. The autofix gate checks the parsed base
  (PromptingProtocol.SUPPORTS_AUTOFIX, True for python) instead of
  protocol.startswith('python:'); the result is the same for every existing
  protocol.
• LfQuery.from_protocol parses the name, then looks up its base and version
  as before. _DEFAULT_PROTOCOL_VERSIONS is unchanged, so a protocol that
  registers its default version there (e.g. markdown) keeps working.
• PromptingProtocol.from_name accepts versions and features (it rejected
  python:2.0), so every schema_repr, value_repr and parse_value call
  takes a recorded name. Instances carry their version, features and full
  name.
• lf.parse dispatches on the base and also accepts names such as
  python:2.0 (it accepted exactly python or json); lf.score accepts
  them on both of its paths; lf.query_reward takes the protocol of the
  response.
• Features are private for now (_ProtocolFeature): a feature declares its
  name, its version, the base protocols and versions it supports, and the
  features it implies.

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