feature(core): add core connection-pool monitoring (implementation, cache)

[Oscarcheng0312]

• I have read the CONTRIBUTING.md guidelines.
• I have registered the PR changes.

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Ⅰ. Describe what this PR did

Add the core connection-pool monitoring implementation (transport-agnostic) for CPM backend.

Domain / Model

• ConnectionPoolInfo: runtime snapshot DTO for pool metrics.

Providers

• DruidConnectionPoolMetrics: collect metrics from Druid.
• HikariConnectionPoolMetrics: collect metrics from HikariCP.

Collector

• DataSourceConnectionPoolCollector: aggregate metrics from multiple providers.

Service

• ConnectionPoolMonitor: periodic collection and read APIs.

Cache

• ConnectionPoolInfoCache: in-memory storage for fast reads.

Integration Hook

• DataSourceProxyEnhancer: lifecycle hook for automatic registration/unregistration of monitored DataSources.

This PR does not include any private protocol / networking code (Netty / gRPC).
Transport (Netty / gRPC / REST) will be integrated in a follow-up PR to keep layering clean.

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Ⅱ. Does this pull request fix one issue?

No. It introduces the core capability for the connection-pool-monitor backend, independent of transport.

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Ⅲ. Why don't you add test cases (unit test/integration test)?

Basic unit tests will be added with the follow-up PR.

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Ⅳ. Describe how to verify it

This PR introduces transport-agnostic core logic only. Verification will be provided by
follow-up unit tests that cover providers (Druid / Hikari), the collector, the monitor, and the cache.
Those tests will be sufficient to validate the functionality end-to-end without requiring a running console.

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Ⅴ. Special notes for reviews

• All new Java files include ASF headers.
• No Netty / gRPC classes are included in this PR; those will live in a separate PR.
• The design keeps providers → collector → monitor → cache → enhancer independent from transport for easy pluggability.
• Package structure may be further refined after backend transport abstraction (e.g., gRPC integration) is finalized.