feat: support custom decode handler

[gxcsoccer]

#116

hessian.registerDecodeHandler('java.math.BigDecimal', function (result) {
  return {
    $class: result.$class,
    $: result.$.value,
  };
});

const o = { $class: 'java.math.BigDecimal', $: { value: '100.06' } };
const buf = hessian.encode(o, '1.0');
const output = hessian.decode(buf, '1.0');
assert(output === '100.06');

[codecov]

Codecov Report

Merging #119 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #119   +/-   ##
=======================================
  Coverage   96.27%   96.27%           
=======================================
  Files          11       11           
  Lines        1129     1129           
  Branches      211      211           
=======================================
  Hits         1087     1087           
  Misses         42       42

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[codecov]

Codecov Report

Merging #119 into master will increase coverage by 0.06%.
The diff coverage is 96.15%.

@@            Coverage Diff             @@
##           master     #119      +/-   ##
==========================================
+ Coverage   96.27%   96.34%   +0.06%     
==========================================
  Files          11       12       +1     
  Lines        1129     1150      +21     
  Branches      211      210       -1     
==========================================
+ Hits         1087     1108      +21     
  Misses         42       42

Impacted Files	Coverage Δ
lib/custom_handler.js	100% <100%> (ø)
lib/v1/decoder.js	98.2% <100%> (ø)	⬆️
index.js	100% <100%> (ø)	⬆️
lib/v2/decoder.js	92.9% <80%> (+0.02%)	⬆️

---

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[denghongcai]

啥时候 merge

[gxcsoccer]

2.9.0