fix: cli performance

[echosoar]

• faas-cli动态加载插件，相关性能对比
  from:

Benchmark:
  faas-cli invoke        x 0.40 ops/sec ±3.09% (5 runs sampled)
  faas-cli help          x 0.54 ops/sec ±5.14% (6 runs sampled)
  faas-cli test          x 0.48 ops/sec ±0.58% (6 runs sampled)

to:

Benchmark:
  faas-cli invoke        x 0.57 ops/sec ±0.71% (6 runs sampled)
  faas-cli help          x 0.54 ops/sec ±0.64% (6 runs sampled)
  faas-cli test          x 0.87 ops/sec ±0.54% (7 runs sampled)

即：
invoke 提升 42.5%
help 无提升
test 提升 81.25%

[codecov-io]

Codecov Report

Merging #92 into master will decrease coverage by 0.08%.
The diff coverage is 68.75%.

@@            Coverage Diff             @@
##           master      #92      +/-   ##
==========================================
- Coverage   80.66%   80.57%   -0.09%     
==========================================
  Files          72       72              
  Lines        2560     2595      +35     
  Branches      239      245       +6     
==========================================
+ Hits         2065     2091      +26     
- Misses        493      502       +9     
  Partials        2        2

Impacted Files	Coverage Δ
packages/serverless-invoke/src/core.ts	79.45% <100%> (+1.07%)	⬆️
packages/faas-cli/src/index.ts	46.29% <66.66%> (-5.56%)	⬇️

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