feat: Add startup hooks

[seeflood]

What this PR does:
Add startup hooks:

• Add an extends field in the config.json for extendibility. See [Proposal] Add an extends field in the config.json for extendibility #383
• Add InitRuntimeStage hook
• Add custom.Component for user custom components. See User custom component #416
• modify user custom api interface
  

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov]

Codecov Report

Merging #405 (f3ca2c2) into main (d74ff0e) will increase coverage by 0.25%.
The diff coverage is 81.81%.

❗ Current head f3ca2c2 differs from pull request most recent head 8467396. Consider uploading reports for the commit 8467396 to get more accurate results

@@            Coverage Diff             @@
##             main     #405      +/-   ##
==========================================
+ Coverage   60.24%   60.50%   +0.25%     
==========================================
  Files         117      118       +1     
  Lines        6268     6357      +89     
==========================================
+ Hits         3776     3846      +70     
- Misses       2130     2142      +12     
- Partials      362      369       +7     

Impacted Files	Coverage Δ
pkg/grpc/default_api/api_pubsub.go	41.93% <ø> (ø)
pkg/grpc/default_api/api_state.go	86.48% <ø> (ø)
pkg/runtime/config.go	60.00% <ø> (ø)
pkg/runtime/options.go	40.35% <80.00%> (+8.43%)	⬆️
pkg/runtime/runtime.go	60.36% <80.00%> (+3.06%)	⬆️
components/custom/registry.go	84.00% <84.00%> (ø)
pkg/grpc/dapr/dapr_api.go	58.53% <100.00%> (ø)
pkg/grpc/default_api/api.go	85.00% <100.00%> (ø)

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[seeflood]

todo:

• add a demo
• add more description in the document https://mosn.io/layotto/#/zh/design/api_plugin/design?id=_22-%e8%ae%be%e8%ae%a1%e7%9b%ae%e6%a0%87

[seeflood]

@zhenjunMa @wenxuwan @MoonShining I added document ,see
https://seeflood.github.io/layotto/#/zh/design/api_plugin/design?id=_24-%e4%bd%bf%e7%94%a8%e6%8c%87%e5%8d%97

https://seeflood.github.io/layotto/#/zh/component_specs/custom/common

https://seeflood.github.io/layotto/#/zh/start/api_plugin/helloworld

[wenxuwan]

LGTM

[seeflood]

Thank you all for your review !