Add variable support to YAML test expression parser

[miguelgrinberg]

While debugging a YAML test failure in 8.19 I found that our parser does not appear to support variables, as opposite to expressions that are based on the response from the last request. Take the expression $profile.uid for example.

• _lookup('$profile.uid') is called
• value is assigned a dict that was a response in a previous operation
• we enter the for-loop with step == '$profile'
• the call to self._resolve(step) returns the value stored in self._state['profile'] and assigns it to step
• the assert step in value fails

The changes to the parser add support for variables such as $profile. The value variable is only initialized to last_response when the expression does not start with a $, which means it is relative to that response. If the expression starts with $ we initialize to None. Then inside the loop we differentiate between a reference to a variable that starts the expression (when value is None) versus one that is in the middle and should be treated as a key lookup in the current value.

The issue that prompted this change is a block of match statements highlighted here: https://github.com/elastic/elasticsearch-clients-tests/blob/8.19/tests/security/50_user_profile.yml#L80-L86, which are currently failing.