all: report client/app name to control plane

[dylan-tailscale]

• Work with the control plane team to add a new field to Hostinfo allowing us to separately report our client name (tailscale-rs) and application name (language binding or whatever the developer specifies). May need to be gated behind a capability version bump.
• Add the (entirely optional) ability to specify the application name via the Rust API and every language binding.
  • This is not a unique identifier. This field will be uniform across all instances of the application, and does not have to be provided, in which case it'll default to the language binding.
• Figure out the best way to determine the total/aggregate number of active tailscale-rs-based applications connecting to the control plane on a daily/monthly basis while preserving user privacy.
  • Maximum granularity is at the application/language-binding level; we do not want metrics about individual users or individual instances of applications.
  • This is mainly to distinguish "which connections are tailscaled, which are tsnet, which are tailscale-rs, etc".