feat: add hooking point for AUTHORIZE

[adrysn]

This PR adds hooking point for AUTHORIZE with the FIRST_COMPLETED requirement.

• The hook accepts params and dbpool parameters. Especially, dbpool is needed to access DB from the hook.
• When any one of AUTHORIZE hook is passed, the user object returned by the hook is used .
  • Thus, each AUTHORIZE hook should ensure that a corresponding Backend.AI account exists.
• For example, LDAP hook may perform the following jobs:
  • Check if the user's login credential (email, password) passes the authentication from LDAP server.
    • If not passed, just reject the request.
  • If passed, ensure a Backend.AI account with the same email exists.
    • If it does not exist, create a new Backend.AI account
  • Return the existing or new user object.
• If there is no AUTHORIZE hook in the manager, just performs normal check_credential authentication process.

Related LDAP hook implementation: https://github.com/lablup/backend.ai-ldap-plugin/pull/1

[codecov]

Codecov Report

Merging #339 into master will decrease coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #339      +/-   ##
==========================================
- Coverage   47.56%   47.51%   -0.05%     
==========================================
  Files          48       48              
  Lines        7491     7498       +7     
==========================================
  Hits         3563     3563              
- Misses       3928     3935       +7     

Impacted Files	Coverage Δ
src/ai/backend/gateway/auth.py	52.40% <0.00%> (-1.07%)	⬇️

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