[Feature] Allow to lookup Node by its Tailnet IP address in GRPC API

[jirutka]

Use case

For monitoring purposes, it’s necessary to find out to which node and user a given IP address belongs.

Description

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Contribution

• I can write the design doc for this feature
• I can contribute this feature

How can it be implemented?

a. Change GetNodeRequest to oneof node_id or tailnet_ip. If tailnet_ip is provided, lookup Node by its IPv4 or IPv6 address.
b. Add a new method, e.g. GetNodeByIp.

[kradalby]

Sounds reasonable to me, maybe make it reasonable generic and tap into the "list" version so we can do id, name, ip etc?

[kradalby]

I started look at this based while working on #2246 and it looks like our usage of the grpc web api generator we use prevents us from using oneof: grpc-ecosystem/grpc-gateway#441