feat: courier template configs

[Benehiko]

Related issue(s)

Implements: #2054

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
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  [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 #2156 (6f766f5) into v0.8 (03b687f) will increase coverage by 0.14%.
The diff coverage is 89.25%.

@@            Coverage Diff             @@
##             v0.8    #2156      +/-   ##
==========================================
+ Coverage   75.70%   75.84%   +0.14%     
==========================================
  Files         295      296       +1     
  Lines       15715    15858     +143     
==========================================
+ Hits        11897    12028     +131     
- Misses       2963     2971       +8     
- Partials      855      859       +4     

Impacted Files	Coverage Δ
selfservice/strategy/link/sender.go	71.27% <50.00%> (ø)
driver/config/config.go	81.89% <71.42%> (-0.54%)	⬇️
courier/template/load_template.go	74.50% <82.00%> (+7.84%)	⬆️
courier/template/testhelpers/testhelpers.go	95.06% <95.06%> (ø)
courier/courier.go	69.44% <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)
... and 4 more

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[aeneasr]

Thank you! This looks really solid. A few follow ups:

• Could you please add documentation to the courier section which explains how to customize individual templates?
• Is it possible to load only one template from a custom source, and then all other templates are used from the default source?
• Could you customize the recovery or verification e2e test and set one custom template and then, in the respective e2e test, ensure that the custom template is being used? You can add one or two custom templates to here and then assert the contents around here. Alternatively you could also update the config on demand in the e2e test and write an individual test just for the custom template as e2e. To set a custom config value within your test, see here and subsuquently here. I recommend using base64 based template loading to make your life easier.
• Can you please add a schema test to verify that your JSON Schema is correct?

[aeneasr]

@Benehiko are you waiting for a review on this one?

[Benehiko]

I'm still writing e2e tests, it's almost ready

[Benehiko]

Should be good to go on green CI