feat: resolve placeholder

[ZLBer]

What this PR does:
resolve placeholder ${xxx:defaultValue}
Which issue(s) this PR fixes:
Fixes #1983

You should pay attention to items below to ensure your pr passes our ci test
We do not merge pr with ci tests failed

• All ut passed (run 'go test ./...' in project root)
• After go-fmt ed , run 'go fmt project' using goland.
• Golangci-lint passed, run 'sudo golangci-lint run' in project root.
• After import formatted, (using imports-formatter to run 'imports-formatter .' in project root, to format your import blocks, mentioned in CONTRIBUTING.md above)
• Your new-created file needs to have apache license at the top, like other existed file does.
• All integration test passed. You can run integration test locally (with docker env). Clone our dubbo-go-samples project and replace the go.mod to your dubbo-go, and run 'sudo sh start_integration_test.sh' at root of samples project root. (M1 Slice is not Support)

[zhaoyunxing92]

很不错的PR,不过建议你可以看下这个代码：config#L33-L70 可以递归解析还可以设置默认值eg:${dubbo.provider.filter:nacos}

[codecov-commenter]

Codecov Report

Merging #2014 (fb92c94) into master (1d6b35f) will increase coverage by 0.27%.
The diff coverage is 93.10%.

@@            Coverage Diff             @@
##           master    #2014      +/-   ##
==========================================
+ Coverage   44.40%   44.67%   +0.27%     
==========================================
  Files         283      283              
  Lines       17067    17133      +66     
==========================================
+ Hits         7578     7654      +76     
+ Misses       8687     8672      -15     
- Partials      802      807       +5     

Impacted Files	Coverage Δ
config/config_resolver.go	67.27% <93.10%> (+26.53%)	⬆️
cluster/cluster/available/cluster_invoker.go	66.66% <0.00%> (-6.67%)	⬇️
config/graceful_shutdown_config.go	89.83% <0.00%> (-2.90%)	⬇️
metrics/prometheus/reporter.go	30.25% <0.00%> (-1.54%)	⬇️
common/url.go	59.04% <0.00%> (-0.27%)	⬇️
cluster/router/tag/router.go	59.64% <0.00%> (ø)
cluster/cluster/zoneaware/cluster_interceptor.go	40.00% <0.00%> (ø)
metadata/definition/definition.go	70.66% <0.00%> (+7.87%)	⬆️
registry/zookeeper/service_discovery.go	17.07% <0.00%> (+15.85%)	⬆️

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