add cli to pull gateways that location has changed since given timestamp

[bbalser]

Add gateway location-changed-at CLI command to mobile-config-cli

Summary

Adds a new location-changed-at subcommand under mobile-config-cli gateway that streams all hotspots whose location changed at or after a given timestamp, using the gateway_info_stream_req_v4 RPC.

Usage

mobile-config-cli gateway location-changed-at --min-location-changed-at <UNIX_SECONDS> [--batch-size 5000]

Outputs pretty JSON with the filter value, a match count, and per-gateway details: name, pubkey, device_type, location/lat/lon, location_changed_at, num_location_asserts, created_at, updated_at, owner, owner_changed_at.

Changes

• cmds/mod.rs — new LocationChangedAt command variant + args struct (--min-location-changed-at, optional --batch-size default 5000, global keypair/host/pubkey).
• client.rs — new info_stream_v4 method on GatewayClient that builds, signs, and streams a GatewayInfoStreamReqV4 (min_location_changed_at set; other filters left at 0); registered impl_sign!/impl_verify! for the V4 req/res types.
• cmds/gateway.rs — serializable GatewayInfoV4 summary with TryFrom<GatewayInfoV4Proto>, plus the location_changed_at handler.
• main.rs — wired the new subcommand into dispatch.

Notes

• Server filter semantics are "location changed at or after" the given timestamp (matches the proto's min_location_changed_at field).
• cargo check and cargo clippy pass clean for mobile-config-cli.