Add a cli option to get a witness' checkpoints

[gregoire-mullvad]

Add a new -w option that takes a witness name and returns the latest checkpoints signed by this witness
This is useful for witness admin (or ArmoredWitness custodians) to check that their witness is working.

Usage:

$ go run ./cmd/client -w ArmoredWitness-dry-sunset
Fetching checkpoints from distributor...
╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾╾
Log "transparency.dev-aw-ftlog-ci-3" (0b963db2162e516836b4afbf8ff75aca120af717d3b93a332464cd5af4146e99)
✅ Got checkpoint:

transparency.dev/armored-witness/firmware_transparency/ci/3
59
kOFMkeamIdrYFLx5zbWFhflak6dlEEDOU++fxYEZMBw=

— transparency.dev-aw-ftlog-ci-3 P2iVIt3Jsaha3E2i0MHdKZ6fgYJTgXO86nUsxKPVq6uTwRSclTSh/ELnDGbNL+XDyD3EkWIdL2AGGNmbN7IN2rB/7gE=
— ArmoredWitness-dry-sunset uS6j90LXi2YAAAAAyvMVEmnYJJ8c4ohhwW9YeG6yXgnGEmC8AB0K7LDFweW2hXQloeUGerCO86ei+NKR7/DYXkxQFh1J6pXr0oXDAQ==

Witness timestamps:
— ArmoredWitness-dry-sunset: 1 month ago (2024-07-08T14:10:42+02:00)
[...]

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@b51e407). Learn more about missing BASE report.

Files	Patch %	Lines
cmd/client/client.go	0.00%	12 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #202   +/-   ##
=======================================
  Coverage        ?   30.71%           
=======================================
  Files           ?        8           
  Lines           ?      534           
  Branches        ?        0           
=======================================
  Hits            ?      164           
  Misses          ?      341           
  Partials        ?       29           

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[AlCutter]

Thanks!