Runner: provide standardized parsing of Arazzo runtime expressions

[char0n]

The goal of this issue is to provide standardized parsing of Arazzo runtime expressions. Arazzo Runtime expressions has a standardized ABNF syntax, so we need ABNF parser for that.

  expression = ( "$url" / "$method" / "$statusCode" / "$request." source / "$response." source / "$inputs." name / "$outputs." name / "$steps." name / "$workflows." name / "$sourceDescriptions." name / "$components." name / "$components.parameters." parameter-name)
      parameter-name = name ; Reuses 'name' rule for parameter names
      source = ( header-reference / query-reference / path-reference / body-reference )
      header-reference = "header." token
      query-reference = "query." name
      path-reference = "path." name
      body-reference = "body" ["#" json-pointer ]
      json-pointer    = *( "/" reference-token )
      reference-token = *( unescaped / escaped )
      unescaped       = %x00-2E / %x30-7D / %x7F-10FFFF
         ; %x2F ('/') and %x7E ('~') are excluded from 'unescaped'
      escaped         = "~" ( "0" / "1" )
        ; representing '~' and '/', respectively
      name = *( CHAR )
      token = 1*tchar
      tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "." /
        "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA

Most popular parsers are:

• https://github.com/ldthomas/apg-py
• https://pypi.org/project/abnf/

I'd use apg-py for now as apg parsing toolset is something that I'm more familiar with and actively using it in other JavaScript/TypeScript based projects.