Support custom formatters in custom request/response headers

Envoy's standard command operators can be used in the value for custom request/response headers. Please also support custom formatters (or custom command operators?) in addition to the built-in formatters.

Custom formatters are already supported in the access log configuration (see "formatters" in config.core.v3.SubstitutionFormatString), so maybe this can be implemented in a similar way.

Example:

route_config:
  name: local_route
  request_headers_to_add:
  - header:
    key: "x-my-header"
    value: "%MY_CUSTOM_COMMAND_OPERATOR%"