Path Uri Template Mixed Variable Literals

Added support for mixed variable/literal path segments in URI template matching and rewriting, enabling patterns such as /api/v{version}/users/{id}.json where a variable is embedded within a path segment alongside literal text. The surrounding prefix and suffix must match literally while only the variable portion is captured. This is controlled by runtime guard envoy.reloadable_features.uri_template_mixed_variable_literals, which is enabled by default.