feat(cli): add p2p diagnostics to ping (#14426)

First PR to address #14244.

Adds common potential reasons as to why a direct connection to the workspace agent couldn't be established to `coder ping`:
- If the Coder deployment administrator has blocked direction connections (`CODER_BLOCK_DIRECT`).
- If the client has no STUN servers within it's DERP map.
- If the client or agent appears to be behind a hard NAT, as per Tailscale `netInfo.MappingVariesByDestIP`

Also adds a warning if the client or agent has a network interface below the 'safe' MTU for tailnet. This warning is always displayed at the end of a `coder ping`.

codersdk/healthsdk/healthsdk.go

@@ -273,3 +273,10 @@ type ClientNetcheckReport struct {
 	DERP       DERPHealthReport `json:"derp"`
 	Interfaces InterfacesReport `json:"interfaces"`
 }
+
+// @typescript-ignore AgentNetcheckReport
+type AgentNetcheckReport struct {
+	BaseReport
+	NetInfo    *tailcfg.NetInfo `json:"net_info"`
+	Interfaces InterfacesReport `json:"interfaces"`
+}