feat: add dynamic target support

[ylighgh]

Summary

• add a /probe endpoint for MongoDB targets supplied at scrape time
• load credentials and TLS options from named YAML auth modules
• reject credentials, paths, and query parameters in dynamic targets
• redact credentials from connection URI logs
• document dynamic target configuration and the new --config.file flag

Testing

• make format
• go test -tags gssapi -count=1 . -run '^Test(LoadAuthConfig|LoadAuthConfigRejectsInvalidModule|BuildDynamicURI|ResolveAuthModule|ValidateTargetConfiguration|DynamicTargetFactoryRejectsUnknownModule|RedactMongoURI)$'
• go test -tags gssapi -count=1 ./exporter -run '^Test(DynamicTarget|DynamicTargetValidation|DynamicOnlyServer|ServerRequiresStaticOrDynamicTarget)$'
• go build .
• go run .github/check-license.go

The full integration suite requires the repository's MongoDB Docker sandbox; GitHub Actions runs that matrix.

Reviewer notes

The dynamic exporter handlers are cached by auth module and target so repeated Prometheus scrapes reuse MongoDB clients.

[it-percona-cla]

All committers have signed the CLA.