ctclient: Use Cobra library for command-line tools

[pav-kv]

This change refactors the ctclient tool to use the widely adopted Cobra library
for handling CLI flags and subcommands.

Before:

$ ./ctclient -h
Usage of ./ctclient:
  -align_getentries
        Enable get-entries request alignment (default true)
  -alsologtostderr
        log to standard error as well as files
  -cert_chain string
        Name of file containing certificate chain as concatenated PEM files
  -chain
        Display entire certificate chain
  -first int
        First entry to get (default -1)
  -last int
        Last entry to get (default -1)
  -leaf_hash string
        Leaf hash to retrieve (as hex string or base64)
  -log_backtrace_at value
        when logging hits line file:N, emit a stack trace
  -log_dir string
        If non-empty, write log files in this directory
  -log_list string
        Location of master log list (URL or filename) (default "https://www.gstatic.com/ct/log_list/all_logs_list.json")
  -log_mmd duration
        Log's maximum merge delay (default 24h0m0s)
  -log_name string
        Name of log to retrieve information from --log_list for
  -log_uri string
        CT log base URI (default "https://ct.googleapis.com/rocketeer")
  -logtostderr
        log to standard error instead of files
  -prev_hash string
        Previous tree hash to check against (as hex string or base64)
  -prev_size uint
        Previous tree size to get consistency against
  -pub_key string
        Name of file containing log's public key
  -size uint
        Tree size to query at
  -skip_https_verify
        Skip verification of HTTPS transport connection
  -stderrthreshold value
        logs at or above this threshold go to stderr
  -text
        Display certificates as text (default true)
  -timestamp int
        Timestamp to use for inclusion checking
  -tree_hash string
        Tree hash to check against (as hex string or base64)
  -v value
        log level for V logs
  -vmodule value
        comma-separated list of pattern=N settings for file-filtered logging

After:

$ ./ctclient --help
A command line client for Certificate Transparency logs

Usage:
  ctclient [command]

Available Commands:
  bisect                Find a log entry by timestamp
  completion            Generate the autocompletion script for the specified shell
  get-consistency-proof Fetch and verify a consistency proof between two tree states
  get-entries           Fetch a range of entries in the log
  get-inclusion-proof   Fetch and verify the inclusion proof for an entry
  get-roots             Fetch the root certificates accepted by the log
  get-sth               Fetch the latest STH of the log
  help                  Help about any command
  upload                Submit a certificate (pre-)chain to the log

Flags:
...
Use "ctclient [command] --help" for more information about a command.

$ ./ctclient getentries --help
Fetch a range of entries in the log

Usage:
  ctclient get-entries {--log_uri uri | --log_name name [--log_list {file|uri}]} [--pub_key file] --first=idx [--last=idx] [flags]

Aliases:
  get-entries, getentries, entries

Flags:
      --chain       Display entire certificate chain
      --first int   First entry to get (default -1)
  -h, --help        help for get-entries
      --last int    Last entry to get (default -1)
      --text        Display certificates as text (default true)

Global Flags:
      --alsologtostderr                  log to standard error as well as files
      --log_backtrace_at traceLocation   when logging hits line file:N, emit a stack trace (default :0)
      --log_dir string                   If non-empty, write log files in this directory
      --log_list string                  Location of master log list (URL or filename) (default "https://www.gstatic.com/ct/log_list/all_logs_list.json")
      --log_name string                  Name of log to retrieve information from --log_list for
      --log_uri string                   CT log base URI (default "https://ct.googleapis.com/rocketeer")
      --logtostderr                      log to standard error instead of files
      --pub_key string                   Name of file containing log's public key
      --skip_https_verify                Skip verification of HTTPS transport connection
      --stderrthreshold severity         logs at or above this threshold go to stderr (default 2)
  -v, --v Level                          log level for V logs
      --vmodule moduleSpec               comma-separated list of pattern=N settings for file-filter

Part of #900

Checklist

• I have updated the CHANGELOG.
  • Adjust the draft version number according to semantic versioning rules.
• I have updated documentation accordingly.

[pav-kv]

I recommend reviewing commit-by-commit. For now this is a structural change, with no behaviour changes (except the look of help hints).

Next steps will be refactoring each command, and improving usability.

[AlCutter]

Just a couple of nits - LVGTM in the grand scheme of things, thanks for doing this!

[pav-kv]

TODO in this PR:

• A few typos.
• glog flags.

[pav-kv]

Ping

[pav-kv]

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