Add --serial for connecting only to one specific TKey

[quite]

Description

When --serial is used, the agent will support multiple TKeys being plugged in, and will only use one which has a matching serial number (if any).

Type of change

Please tick any that are relevant to this PR and remove any that aren't.

• Bugfix (non breaking change which resolve an issue)
• Feature (non breaking change which adds functionality)
• Breaking Change (a change which would cause existing functionality to not work as expected)
• Documentation (a change to documentation)

Submission checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my changes
• I have tested and verified my changes on target
• My changes are well written and CI is passing
• I have squashed my work to relevant commits and rebased on main for linear history
• I have added a "Co-authored-by: x" if several people contributed, either pair programming or by squashing commits from different authors.
• I have updated the documentation where relevant (readme, dev.tillitis.se etc.)
• QEMU is updated to reflect changes

[quite]

Ugh, golangci-lint complains: "Error: cmd/tkey-ssh-agent/main.go:42:1: cyclomatic complexity 31 of func main is high (> 30) (gocyclo)". Yeah given the current design of main(), that is going to happen as new flags are added...

[mchack-work]

At a first glance it's a reasonable change. I like it. Can't promise to do a real review very soon, though.