refactor(coredhcp/coresmd): separate functionality into separate packages

[synackd]

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Description

Refactor tftp, cache, iface, and smdclient from the main CoreSMD package into their own. Plugin behavior should remain the same, this is just a code refactor.

(This is in preparation for an upcoming hostname rule feature)

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Refactor

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