feat: remove duplicate queries during settings flow and use better index hint for credentials lookup

[aeneasr]

This patch reduces duplicate GetIdentity queries as part of submitting the settings flow, and improves an index to significantly reduce credential lookup.

For better debugging, more tracing ha been added to the settings module.

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
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  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
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Further Comments

[codecov]

Codecov Report

Attention: Patch coverage is 74.50199% with 64 lines in your changes missing coverage. Please review.

Project coverage is 78.60%. Comparing base (215af57) to head (8087a0e).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
selfservice/strategy/oidc/strategy_settings.go	61.22%	18 Missing and 1 partial ⚠️
selfservice/flow/settings/error.go	64.51%	7 Missing and 4 partials ⚠️
selfservice/flow/settings/handler.go	77.41%	4 Missing and 3 partials ⚠️
selfservice/flow/settings/hook.go	79.31%	4 Missing and 2 partials ⚠️
selfservice/strategy/totp/settings.go	71.42%	4 Missing and 2 partials ⚠️
selfservice/strategy/lookup/settings.go	75.00%	2 Missing and 1 partial ⚠️
selfservice/strategy/password/settings.go	72.72%	2 Missing and 1 partial ⚠️
selfservice/strategy/webauthn/settings.go	81.25%	2 Missing and 1 partial ⚠️
persistence/sql/identity/persister_identity.go	66.66%	0 Missing and 2 partials ⚠️
selfservice/strategy/link/strategy_recovery.go	75.00%	1 Missing and 1 partial ⚠️
... and 1 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4193      +/-   ##
==========================================
+ Coverage   78.52%   78.60%   +0.07%     
==========================================
  Files         380      380              
  Lines       27176    27128      -48     
==========================================
- Hits        21340    21324      -16     
+ Misses       4215     4189      -26     
+ Partials     1621     1615       -6     

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[aeneasr]

@alnr the old index is not dropped yet (identity_credential_identifiers_nid_i_ici_idx), will be done in the next release

[aeneasr]

@alnr i changed the index structure to use identity_credential_id as the prefix (it is also the highest cardinality) and then we can use it both for FK and the query credentials

[aeneasr]

i is never nil