feat(yarn_client): SSL/TLS config for YARN REST connections

Add per-Connection ssl_verify / ssl_ca_bundle plus global defaults so
CDH 5 / on-prem clusters with self-signed certs or custom CA bundles can
be queried without patching code.

- Connection gets ssl_verify (bool|None) and ssl_ca_bundle (str|None)
- Settings gets ssl_verify_default and ssl_ca_bundle_default
- New YarnClientConfig dataclass carries the resolved verify= value
- _request passes verify= through to httpx.request
- All public yarn_client functions now take YarnClientConfig instead of
  a bare yarn_rm_url string; tool call sites resolve the Connection
- SaveConnectionRequest exposes the two new fields

Tests cover per-connection CA bundle, per-connection verify=False,
global default fallback, and connection-not-found error.

spark_executor/tools/requests.py

@@ -22,6 +22,8 @@ class SaveConnectionRequest(BaseModel):
     deploy_mode: str = "cluster"
     yarn_rm_url: str | None = None
     spark_conf: dict[str, str] | None = None
+    ssl_verify: bool | None = None
+    ssl_ca_bundle: str | None = None
 
 
 class PrepareSubmitJobRequest(BaseModel):