feat(db): honor an optional init_command on every connection pool

[raghav-reglobe]

What

Add an optional init_command to DatabaseConfig (global and token-bound), carry it in DatabasePoolConfig as a NotRequired key, and pass it to aiomysql.create_pool in _create_pool_with_candidates.

Why

Hosts that front Doris with a service account often need every pooled connection to start in a prepared session state: a session-narrowing switch, a SET of session variables, a workload-group pin. aiomysql's init_command is the right hook (it runs once per new connection), but the pool factory builds every pool without it. Today a host has to subclass DorisConnectionManager and re-implement pool creation just to add one statement, and then keep that subclass in lockstep with every refactor of the factory. On 0.6.1, where pool creation lived in six methods, such a subclass silently missed paths; 1.0.0 collapsed the factory into one method, which makes a config field the durable seam.

Behaviour

• Unset (default): the key is omitted from the pool config, so existing pool configs stay byte-identical for equality-based callers and serialized configs.
• Set on the global config: applies to the global pool; per-user pools inherit it.
• Set on a token's database_config: applies to that token's pool.

Tests

test/utils/test_pool_init_command.py: global and token-bound paths pass the statement through, the default omits the key, an explicit pool config takes precedence. Existing pool tests unchanged.