- Add current node signature to `ipnstate.NetworkLockStatus`;
- Print current node signature in a human-friendly format as part
of `tailscale lock status`.
Examples:
```
$ tailscale lock status
Tailnet lock is ENABLED.
This node is accessible under tailnet lock. Node signature:
SigKind: direct
Pubkey: [OTB3a]
KeyID: tlpub:44a0e23cd53a4b8acc02f6732813d8f5ba8b35d02d48bf94c9f1724ebe31c943
WrappingPubkey: tlpub:44a0e23cd53a4b8acc02f6732813d8f5ba8b35d02d48bf94c9f1724ebe31c943
This node's tailnet-lock key: tlpub:44a0e23cd53a4b8acc02f6732813d8f5ba8b35d02d48bf94c9f1724ebe31c943
Trusted signing keys:
tlpub:44a0e23cd53a4b8acc02f6732813d8f5ba8b35d02d48bf94c9f1724ebe31c943 1 (self)
tlpub:6fa21d242a202b290de85926ba3893a6861888679a73bc3a43f49539d67c9764 1 (pre-auth key kq3NzejWoS11KTM59)
```
For a node created via a signed auth key:
```
This node is accessible under tailnet lock. Node signature:
SigKind: rotation
Pubkey: [e3nAO]
Nested:
SigKind: credential
KeyID: tlpub:6fa21d242a202b290de85926ba3893a6861888679a73bc3a43f49539d67c9764
WrappingPubkey: tlpub:3623b0412cab0029cb1918806435709b5947ae03554050f20caf66629f21220a
```
For a node that rotated its key a few times:
```
This node is accessible under tailnet lock. Node signature:
SigKind: rotation
Pubkey: [DOzL4]
Nested:
SigKind: rotation
Pubkey: [S/9yU]
Nested:
SigKind: rotation
Pubkey: [9E9v4]
Nested:
SigKind: direct
Pubkey: [3QHTJ]
KeyID: tlpub:44a0e23cd53a4b8acc02f6732813d8f5ba8b35d02d48bf94c9f1724ebe31c943
WrappingPubkey: tlpub:2faa280025d3aba0884615f710d8c50590b052c01a004c2b4c2c9434702ae9d0
```
Updates tailscale/corp#19764
Signed-off-by: Anton Tolchanov <anton@tailscale.com>