feat(field): add zap.Dict to Field constructor list

[hhk7734]

When I want to log a sub dictionary, it's hard to find that I can just log zap.Object("k", ObjectMarshalerFunc(func(inner ObjectEncoder) error { ... })), and it seems unintuitive.

So I added zap.Dict.

Example

zap.Dict("1",
	zap.String("1-1", "b"),
	zap.Bool("1-2", true),
	zap.Dict("1-3",
		zap.String("1-3-1", "d"),
	),
),

{"1": {"1-1": "b", "1-2": true, "1-3": {"1-3-1": "d"}}}

{
  "1": {
    "1-1": "b",
    "1-2": true,
    "1-3": {
      "1-3-1": "d"
    }
  }
}

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #1297 (c19628d) into master (5da8736) will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1297      +/-   ##
==========================================
+ Coverage   97.61%   97.77%   +0.15%     
==========================================
  Files          52       52              
  Lines        3355     3366      +11     
==========================================
+ Hits         3275     3291      +16     
+ Misses         69       65       -4     
+ Partials       11       10       -1     

Files Changed	Coverage Δ
field.go	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[abhinav]

Thanks, @hhk7734. This is a great idea!
I've made some changes to the implementation. Namely, instead of adding a new field type, I've implemented this functionality outside in terms of a new private ObjectMarshaler implementation. That way, the zapcore package doesn't see any changes (except new tests).

The change LGTM, but I'd like to get another maintainer to also look over it.
CC @mway @sywhang @prashantv

[mway]

LGTM - thanks @hhk7734 and @abhinav!