Use a pre-allocated DynamicJsonDocument to more
easily create the JSON structure.  As a nice side
effect, it also performs only two allocations:
One for the DynamicJsonDocument,
One for  `result.reserve()` call.

The old function had 28 lines of the form:
`(String)json += "foo" + (String)bar() + ","`

Each line creates three temporary String objects:
(e.g., "foo", result from bar()), ",").  It then appends
"foo" + "bar" (often another allocation), then appends
the trailing close-quote and comma (unlikely, but
potentially another allocation).  Finally, the first
temporary variable (now "foo : bar,") is appended
to the current final result string (28 allocations).

The anonymous/unnamed namespace ensures no
unintended pollution of the global namespace,
and makes clear what the external API for this file
contains.  This makes it much easier to change
the underlying implementation, as it's guaranteed
that changes internal to the namespace won't
break other source files.

Use of a scoped enum for field type enables compiler warnings.
Specifically, use of a `switch` statement with no `default` case
will generate a compiler warning/error if missing any values.

Tested to generate 100% identical JSON on Fib256.

Confirmed identical JSON output for at least Fib256.

Verified identical JSON output on at least Fib256.