Use a proper parser for the path template syntax

[coryan]

We are using a regular expression to parse the path templates:

google-cloud-rust/generator/internal/genclient/model.go

Line 22 in 61b9d3b

var HTTPPathVarRegex = regexp.MustCompile(`{([a-zA-Z0-9_.]+?)(=[^{}]+)?}`)

However, path templates cannot be parsed by regular expressions. Their grammar uses recursion:

google-cloud-rust/generator/testdata/googleapis/google/api/http.proto

Lines 229 to 234 in 61b9d3b

	// Template = "/" Segments [ Verb ] ;
	// Segments = Segment { "/" Segment } ;
	// Segment = "*" | "**" | LITERAL | Variable ;
	// Variable = "{" FieldPath [ "=" Segments ] "}" ;
	// FieldPath = IDENT { "." IDENT } ;
	// Verb = ":" LITERAL ;

And therefore they form a non-regular language and require more than regular expressions to parse them correctly. Regular expressions will serve us well for a while, as very few services use the more complex forms of this grammar.

A small recursive descent parser can take care of this:

https://github.com/googleapis/google-cloud-cpp/blob/4174d656136f4b849c8a3d327237f3a96be3e003/generator/internal/http_annotation_parser.cc#L230-L250