feat(jwt-auth): parameter to store JWT in the request context

[mikyll]

Description

The changes introduce a new parameter to jwt-auth plugin's route schema: store_in_ctx. If set to true (default is false) the plugin will store the validated JWT object in the request context.

This feature is useful especially for custom plugins. For example, my company is currently using a custom ACL plugin, based on Casbin (lua-casbin), that parses a JWT to extract permissions.

Therefore, I would say this feature is useful at least for two reasons:

• since the JWT can be removed from the request attributes (via hide_credential = true configuration), this provides another way to pass the token, without exposing it;
• this prevents custom plugins from duplicated code (retrieve and parse JWT object), which jwt-auth plugin already does.

Fixes #11281

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)