Add ReportFieldExtension plugin API for report_checks -fields

[dsengupta0628]

Motivation

report_checks -fields {...} currently supports a fixed set of column
keywords (capacitance, slew, fanout, input_pin, net, src_attr).
Tools layered on top of OpenSTA (resizer, clustering passes, etc.) often
want to surface tool-specific per-pin data alongside timing in the same
report. Today that requires patching OpenSTA core for each new column.

What this adds

A plugin interface so external producers can register columns without
modifying OpenSTA core:

  cpp
  class ReportFieldExtension {
   public:
    virtual std::string_view name() const = 0;   // -fields token
    virtual std::string_view title() const = 0;  // column header
    virtual size_t width() const = 0;
    virtual std::string value(const Path*, const Pin*, const Instance*) const = 0;
  };

  // Sta.hh
  void Sta::registerReportFieldExtension(ReportFieldExtension *ext);  // owns ext

Behavior

• report_checks -fields {...} tokens not matching a built-in keyword are
  forwarded to C++ as StringSeq extension_names. ReportPath::setReportFields
  resolves each against registered extensions. Unmatched → STA-0168 warn.
• A ReportField is created once per registered extension (disabled by
  default); setReportFields toggles enable + assigns a buffer slot.
• reportPathLine/reportPath6 call collectExtensionValues(path, pin, inst, buf) per row, which invokes ext->value(...) for each enabled extension-backed field.
• The inner reportLine dispatches via field->extensionIndex() (O(1) per
  field; no name lookup on the hot path).
• Duplicate registration of the same extension name is a silent no-op
  (tolerates re-init in long-lived Sta hosts).
• Zero overhead when no extensions are enabled (enabled_extension_count_
  short-circuits the per-row collect).