chore: make identity schema provider a proper service

[aeneasr]

Related issue(s)

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 the approval (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.

Further Comments

[codecov]

Codecov Report

Attention: Patch coverage is 73.52941% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 78.03%. Comparing base (e5d3b0a) to head (a23eb7e).

Files	Patch %	Lines
schema/schema.go	81.81%	2 Missing and 2 partials ⚠️
driver/registry.go	0.00%	3 Missing ⚠️
driver/registry_default.go	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3908      +/-   ##
==========================================
- Coverage   78.05%   78.03%   -0.03%     
==========================================
  Files         360      362       +2     
  Lines       25276    25296      +20     
==========================================
+ Hits        19730    19739       +9     
- Misses       4035     4046      +11     
  Partials     1511     1511              

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