feat: warn when src-cli is older than the instance's recommended version

[Montana]

Before running any command, src now does a best-effort check of the running version against the version recommended by the configured Sourcegraph instance (the existing /.api/src-cli/version endpoint) and prints a single warning to stderr when it is behind.

The check is fail-open:

• Missing config, network errors, an unreachable instance, or an unparseable
  version all result in no warning.
• A 3s timeout ensures it never noticeably delays the command.
• It writes to stderr, so --json output is unaffected.
• It skips version/help/no-arg invocations, dev builds, and any run with
  SRC_SKIP_VERSION_CHECK set.
• Comparison is limited to major/minor/patch, so prereleases do not produce
  spurious warnings.

Closes #930

[burmudar]

@Montana thanks for your contribution!

I think there are some missing parts here. None of the methods added here are used anywhere?

[burmudar]

@Montana as mentioned in your other PR. Please add your tests and functionality to this PR.

I will close the other two PRs.