feat: expose courier template load

[Benehiko]

Related issue(s)

@aeneasr

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security. vulnerability, I
  confirm that I got green light (please contact
  [email protected]) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further Comments

[codecov]

Codecov Report

Merging #2082 (3f374a6) into master (9cb1810) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2082   +/-   ##
=======================================
  Coverage   74.91%   74.91%           
=======================================
  Files         293      293           
  Lines       15331    15331           
=======================================
  Hits        11485    11485           
  Misses       3015     3015           
  Partials      831      831           

Impacted Files	Coverage Δ
courier/template/load_template.go	66.66% <100.00%> (ø)
courier/template/recovery_invalid.go	100.00% <100.00%> (ø)
courier/template/recovery_valid.go	100.00% <100.00%> (ø)
courier/template/stub.go	100.00% <100.00%> (ø)
courier/template/verification_invalid.go	100.00% <100.00%> (ø)
courier/template/verification_valid.go	100.00% <100.00%> (ø)

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