feat: configurable, reused DB connection pool

[jabbrwcky]

Description of your changes

Every DB client previously opened a new *sql.DB via sql.Open on every query and closed it immediately, so the provider held an effectively unbounded number of short-lived connections against each server.

This PR introduces a process-wide, DSN-keyed connection-pool cache (pkg/clients/pool): managed resources targeting the same server (driver + DSN) now share a single bounded *sql.DB. Pools apply MaxOpenConns / MaxIdleConns / ConnMaxLifetime / ConnMaxIdleTime and are evicted after going idle, so a credential rotation (new DSN) does not leak pools.

The pool is tunable per ProviderConfig / ClusterProviderConfig via a new optional connectionPool block, for all three databases (MySQL, PostgreSQL, MSSQL), cluster- and namespaced-scoped. Omitted fields use defaults: maxOpenConnections=10, maxIdleConnections=5, maxConnLifetime=1h, maxConnIdleTime=10m.

Notable design points for reviewers:

• The xsql.DB interface intentionally gains no Close(): the cache owns pool lifecycle, so the per-controller Disconnect methods stay no-ops.
• Pools are shared per (driver, DSN); the config that first creates a pool for a DSN wins (in practice a DSN maps to one ProviderConfig).
• Defaults are applied per-field and on a nil connectionPool, so existing ProviderConfigs keep working unchanged.

See docs/connection-pool.md for details.

Fixes #195

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable to ensure this PR is ready for review.

How has this code been tested

• New unit tests for the pool cache (reuse per DSN, distinct pools per driver/DSN, idle-sweep eviction, active pools not swept).
• Updated client and controller unit tests for the new constructor signatures.
• make and make reviewable pass (generate is a no-op diff, lint and full go test ./... are green).

🤖 Generated with Claude Code

[jabbrwcky]

Superseded by #434 (same branch got wiped by a bad rebase, GitHub wouldn't let this PR reopen after recovery).