cmd: rearrange snap-fde-keymgr so so it can be reused by snap-bootstrap

[kubiko]

• rearrange fdeKeyMgr implementation so it can be reused by snap-bootstrap
• add fdekeymgr functionality to snap-bootstrap

fdekeymgr is used during factory reset. Rearranging implementation so snap-fdekeymgr functionality can be built as part of the snap-boostrap, so we can handle factory reset within snap-bootstrap without need to include fdekeymgr binary.

[pedronis]

this seems to do more that what the summary says. About dealing with hooks we'll need to see what is our final design

[kubiko]

this seems to do more that what the summary says. About dealing with hooks we'll need to see what is our final design

@pedronis I have rework PR and updated the description so it represents what PR actually do.

[codecov-commenter]

Codecov Report

Attention: 47 lines in your changes are missing coverage. Please review.

Comparison is base (c0ade98) 78.88% compared to head (830847e) 78.87%.

Files	Patch %	Lines
cmd/snap-fde-keymgr/fdekeymgr/keymanager.go	71.07%	24 Missing and 11 partials :warning:
cmd/snap-bootstrap/cmd_fdekeymgr.go	64.28%	7 Missing and 3 partials :warning:
cmd/snap-fde-keymgr/main.go	50.00%	2 Missing :warning:

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Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12641      +/-   ##
==========================================
- Coverage   78.88%   78.87%   -0.01%     
==========================================
  Files        1030     1032       +2     
  Lines      130562   130596      +34     
==========================================
+ Hits       102993   103010      +17     
- Misses      21161    21175      +14     
- Partials     6408     6411       +3     

Flag	Coverage Δ
unittests	78.87% <69.28%> (-0.01%)	:arrow_down:

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[kubiko]

Hi @chrisccoulson Can you please have a look at this and how this would be affected by the future key configuration and management?

[xnox]

Please note due to size limits we typically desire to have "the one" golang binary in the initrd.