Add support for TFTP single port mode

[rainest]

Add support for TFTP single port mode, which addresses the issue described in OpenCHAMI/deployment-recipes#86 (comment). With that configured, I can successfully retrieve resources over a LoadBalancer using the draft chart at OpenCHAMI/deployment-recipes#87 (comment)

Updates the pin/tftp dependency to v3. The new major was housekeeping for Go module stuff(see https://github.com/pin/tftp/releases/tag/v3.0.0), and is where all ongoing development will land upstream AFAICT. v3 is necessary for single port mode support.

Refactor the plugin wrapper server construction to let us add new settings more cleanly and clean up several constants.

This results in a breaking change because of coredhcp plugin configuration requiring explicit values for all fields.

[rainest]

Originally in OP:

Can we shove arbitary objects under the plugin keys? Something like:

plugins:                                                                                                                                                                                                                                                                                 
    # Base CoreDHCP config                                                                                                                                                                                                                                                                 
   - server_id: 192.168.0.254                                                                                                                                                                                                                                                             
    - dns: 1.1.1.1 1.0.0.1
    - coresmd:
        bssUrl: http://example.com/bss
        smdUrl: http://example.com/smd

would be a lot more flexible regarding defaults, and would be easier to parse into a matching struct. Sticking with the positionals for now for a somewhat less breaking change.

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No, sadly we cannot: the setup4() and setup6() signatures for the plugin interface can only pass []string, so we can't do arbitrary objects.

We could consider KV pairs, like bssURL=http://example.com/bss smdUrl=http://example.com/smd ... with the minor limitation that we'd be unable to use values with = easily, which is probably fine.