Add compatibility with tink

[kqdeng]

(Sorry I made a branch directly off of the main repo...)

Changes:

• Added an envvar HARDWARE_DATA_MODEL to switch between the old cacher model and the new tinkerbell model
  • if envvar is unset/empty/anything other than tinkerbell, it will default to using the old cacher data model
• Abstracted exportedHardware among others to take into account new data model

TODO:

• remove the replace in go.mod pointing to my fork (after the pr in tink gets merged)
• test Subscribe method (honestly, not sure how)
• health check tink when creating new tink client?

  hegel/main.go

  Line 145 in ec7c033

  // add health check for tink?

Related PRs:
boots: tinkerbell/smee#27
tink: tinkerbell/tink#64

[yetang-equinix]

why this id is hard coded here? and the same comments go to other hard coded size and numbers in this structure.

[kqdeng]

This is a mock of what would be returned from cacher so that in the tests we don't have to actually make calls to cacher. I'm working on moving these constants into the actual test file so it's not confusing like so.

[yetang-equinix]

if just for testing, fine for now.

[yetang-equinix]

For coding style related comments, like more informational file header and function header comments, we can do that later. But every time we touch a file or a function is an opportunity to add those.