feat(core): Add ability to verify connector credentials before integrating the connector

[ThisIsMani]

Type of Change

• Bugfix
• New feature
• Enhancement
• Refactoring
• Dependency updates
• Documentation
• CI/CD

Description

This PR adds a new route /account/connectors/verify.

Additional Changes

• This PR modifies the API contract
• This PR modifies the database schema
• This PR modifies application configuration/environment variables

Motivation and Context

To enable merchants to test their credentials before integrating the connector.

How did you test it?

Postman.

curl --location 'http://localhost:8080/account/connectors/verify' \
--header 'Content-Type: application/json' \
--header 'Authorization: Bearer JWT Token' \
--data '{
  "connector_name": "stripe",
  "connector_account_details": {
    "auth_type": "HeaderKey",
    "api_key": "some secret"
  }
}'

curl --location 'http://localhost:8080/account/connectors/verify' \
--header 'Content-Type: application/json' \
--header 'Authorization: Bearer JWT Token' \
--data '{
  "connector_name": "paypal",
  "connector_account_details": {
    "auth_type": "BodyKey",
    "api_key": "some secret",
    "key1": "some other secret"
  }
}'

Checklist

• I formatted the code cargo +nightly fmt --all
• I addressed lints thrown by cargo clippy
• I reviewed the submitted code
• I added unit tests for my changes where possible
• I added a CHANGELOG entry if applicable

[vspecky]

Remove the ConnectorAuthType type from the router crate if you're re-defining it here.

[ThisIsMani]

Making this change will effect a lot of files which are out of scope of this PR. Will be picking this up in a separate PR. This issue tracks it #3018.