Add JSON field path extraction for params/metrics (ticket 015)

RegexToMetricPrimitiveMatcher and the standalone param declaration
(DeclareParamRequest/DataValue::ParamDeclaration) gain an optional
json_field_path, an alternative to capture_group that reads a
dot-separated field path (e.g. "foo.bar") out of lines that parse as
JSON objects. It's additive and serde-defaulted, so existing
regex-based params/metrics are unaffected. Params.tsx now offers a
"JSON field" extraction option when the message match regex looks
like a JSON object.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

src/dto.rs

@@ -20,6 +20,8 @@ pub struct ParamDeclarationDto {
     pub name: String,
     pub message_match_regex: String,
     pub capture_group: String,
+    /// See `crate::data_value::RegexToMetricPrimitiveMatcher::json_field_path`.
+    pub json_field_path: Option<String>,
     pub kind: MetricPrimitiveKind,
 }