feat: add certificate based authentication for smtp client

[alexGNX]

This is an implementation of certificate based authentication for the smtp client, as described in design document #2350

• 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
  security@ory.sh) 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.

[codecov]

Codecov Report

Merging #2351 (e1cb6b1) into master (f23ef2f) will decrease coverage by 0.03%.
The diff coverage is 60.00%.

❗ Current head e1cb6b1 differs from pull request most recent head db1bd7b. Consider uploading reports for the commit db1bd7b to get more accurate results

@@            Coverage Diff             @@
##           master    #2351      +/-   ##
==========================================
- Coverage   76.54%   76.50%   -0.04%     
==========================================
  Files         315      318       +3     
  Lines       17320    17316       -4     
==========================================
- Hits        13257    13248       -9     
- Misses       3127     3132       +5     
  Partials      936      936              

Impacted Files	Coverage Δ
courier/smtp.go	67.79% <50.00%> (+7.21%)	⬆️
driver/config/config.go	81.87% <100.00%> (+0.12%)	⬆️
cmd/jsonnet/lint.go	31.57% <0.00%> (-16.43%)	⬇️
cmd/jsonnet/format.go	37.50% <0.00%> (-12.50%)	⬇️
cmd/identities/list.go	64.51% <0.00%> (-8.66%)	⬇️
cmd/root.go	66.66% <0.00%> (-7.25%)	⬇️
cmd/identities/validate.go	62.12% <0.00%> (-3.64%)	⬇️
cmd/identities/import.go	78.26% <0.00%> (-3.56%)	⬇️
persistence/sql/persister_courier.go	85.00% <0.00%> (-3.34%)	⬇️
session/test/persistence.go	98.61% <0.00%> (-1.39%)	⬇️
... and 7 more

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[aeneasr]

Awesome, thank you for your contribution! This looks pretty good and I have some ideas how to improve it further :)

[aeneasr]

Would it be possible to provide a test for this?

[alexGNX]

Hi! I added some unit tests

[aeneasr]

Awesome, thank you for adding tests! This looks perfect now. As a last item, could you please add a section to the docs, explaining how to use this feature? The perfect place for it would be here:

https://github.com/ory/docs/edit/master/docs/kratos/concepts/email-sms.md

[alexGNX]

@aeneasr Sure, here is the MR for the documentation: 753

[aeneasr]

Awesome, thank you! 🎉 Your contribution makes Ory better :)

[vinckr]

Hello @alexGNX
Congrats on merging your first PR in Ory 🎉 !
Your contribution will soon be helping secure millions of identities around the globe 🌏.
As a small token of appreciation we send all our first time contributors a gift package to welcome them to the community.
Please drop me an email and I will forward you the form to claim your Ory swag!