Add --cert and --key flags to step certificate inspect for mTLS

[DarpanAdhikari]

Name of feature:

--cert/<--key> flags for step certificate inspect to present a TLS client certificate when inspecting a remote server.

Pain or issue this feature alleviates:

Closes #65 — when debugging mTLS-protected environments it was impossible to inspect a server's certificate remotely because the server rejected connections that did not present a valid client certificate.

Why is this important to the project (if not answered above):

step certificate inspect is the primary troubleshooting command for TLS; supporting mTLS peers makes it useful for inspecting certificates inside (or in front of) mTLS-enforced networks.

Is there documentation on how to use this feature? If so, where?

Yes — usage text plus an EXAMPLES section on the inspect command in command/certificate/inspect.go:

$ step certificate inspect https://service.example.com --cert ./client.crt --key ./client.key

In what environments or workflows is this feature supported?

Any remote connection made by inspect (https/tls/tcp/smtps/ldaps) when a client certificate pair is provided. --cert and --key must be used together.

In what environments or workflows is this feature explicitly NOT supported (if any)?

Local (non-URL) certificate/CSR inspection, where no TLS connection is made. Consistent with existing remote-only flags like --roots and --servername.

Supporting links/other PRs/issues:

Fixes #65

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