[Pooler] Per-database TLS config

[levkk]

Allow to configure the following on a per-database entry in pgdog.toml:

[general]
tls_server_ca_certificate = "..."
tls_server_certificate = "..."
tls_server_private_key = "..."
tls_client_ca_certificate = "..."

We should use serde(flatten) and re-use a struct. We then need to "shard" the connector:

pgdog/pgdog/src/net/tls.rs

Line 31 in 18d6b83

static CONNECTOR: ArcSwapOption<ConnectorCacheEntry> = ArcSwapOption::const_empty();