feat(api): add support for agent passthrough api and its config

[zwpaper]

config example:

[agent]
kind = "openai"
api_endpoint = "http://localhost:30889"

[agent.headers]
Authorization = "Bearer API_KEY"

[[agent.tools]]
name = "web_fetch"
description = "Search documents or the web"
available_groups = ["backend"]

[codecov]

Codecov Report

❌ Patch coverage is 6.31579% with 89 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.31%. Comparing base (e19008a) to head (72b7753).

Files with missing lines	Patch %	Lines
crates/tabby/src/routes/agent.rs	0.00%	64 Missing ⚠️
crates/tabby/src/serve.rs	0.00%	13 Missing ⚠️
ee/tabby-webserver/src/service/mod.rs	0.00%	11 Missing ⚠️
ee/tabby-schema/src/schema/mod.rs	0.00%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (6.31%) is below the target coverage (75.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4433      +/-   ##
==========================================
- Coverage   54.35%   54.31%   -0.05%     
==========================================
  Files         227      228       +1     
  Lines       27121    27017     -104     
==========================================
- Hits        14741    14673      -68     
+ Misses      12380    12344      -36     

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