snapd: support FIPS toolchain builds

[xnox]

This adds support for openssl crypto builds using 1.21-fips/stable toolchain.

This build makes TLS communication and snap assertion verification possible using openssl, and thus FIPS openssl.

If such a build is distributed (in .deb and/or .snap), it will do TLS communication with snapstore in a FIPS compliant way; and it will validate assertions in a FIPS compliant way.

This will not setup or do FDE in a FIPS compliant way.

[codecov-commenter]

Codecov Report

Merging #12934 (bc005ef) into master (cea9811) will increase coverage by 0.00%. The diff coverage is n/a.

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@@           Coverage Diff           @@
##           master   #12934   +/-   ##
=======================================
  Coverage   78.79%   78.79%           
=======================================
  Files        1020     1020           
  Lines      127077   127077           
=======================================
+ Hits       100132   100136    +4     
+ Misses      20670    20667    -3     
+ Partials     6275     6274    -1     

Flag	Coverage Δ
unittests	78.79% <ø> (+<0.01%)	:arrow_up:

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Files	Coverage Δ
asserts/crypto.go	72.68% <ø> (ø)
asserts/snap_asserts.go	93.58% <ø> (ø)
boot/assets.go	86.98% <ø> (ø)
cmd/snap/cmd_sign_build.go	71.69% <ø> (ø)
overlord/devicestate/handlers_install.go	64.41% <ø> (ø)

... and 5 files with indirect coverage changes

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