Fix version and schema validation

[jujax]

relative to #11,
I fixed version and priority id in handler.lua as required by Kong 3.9.0.

This pull request includes several updates and improvements to the kong-oidc-auth plugin. The most important changes include updating the version, adding new constants, and refining the schema for better type validation.

Version update and new constants:

• kong-oidc-auth-0.3-0.rockspec was renamed to kong-oidc-auth-0.3.1-0.rockspec and the version was updated to 0.3.1-0.
• src/handler.lua: Added PRIORITY and VERSION constants to the KongOidcAuth object.

Schema improvements:

• src/schema.lua: Enhanced the user_keys field to specify that its elements are of type string.

[CLAassistant]

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