include boot drive hint in instance struct

[mikemrm]

Description

boot_drive_hint is a field provided in cacher to hint at which drive
should be used for the boot drive.

This field already is used by boots when building the vmware config,
however is now being used in osie and needs to be exported.

Reference to boots: https://github.com/tinkerbell/boots/blob/c670ca8c03d5e7c4b951cac56724d21331fb7568/client/instance.go#L42

Why is this needed

Fixes: #

How Has This Been Tested?

How are existing users impacted? What migration steps/scripts do we need?

Checklist:

I have:

• updated the documentation and/or roadmap (if required)
• added unit or e2e tests
• provided instructions on how to upgrade

[codecov]

Codecov Report

Merging #104 (489cf5c) into main (538ebe2) will increase coverage by 2.92%.
The diff coverage is n/a.

❗ Current head 489cf5c differs from pull request most recent head acf4530. Consider uploading reports for the commit acf4530 to get more accurate results

@@            Coverage Diff             @@
##             main     #104      +/-   ##
==========================================
+ Coverage   39.75%   42.68%   +2.92%     
==========================================
  Files           8        8              
  Lines         581      581              
==========================================
+ Hits          231      248      +17     
+ Misses        317      302      -15     
+ Partials       33       31       -2     

Impacted Files	Coverage Δ
hardware/export.go	0.00% <ø> (ø)
grpc/server.go	54.43% <0.00%> (+10.75%)	⬆️

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