Add basic user auth support

[bob-bot]

This PR builds on #107 and should be merged after it.
Closes #108

Add Basic Authentication support to net_http_request table

This PR adds a user_credentials column that accepts username:password format and automatically encodes it as a Basic Auth header, similar to curl's --user flag.

Changes

• Added user_credentials column to table definition
• Added UserCredentials field to request structs
• Implemented automatic Basic Auth header generation with base64 encoding
• Updated documentation with usage examples

Testing

Tested against httpbin.org's basic-auth endpoint:

select
 url,
 response_status_code,
 response_body
from
 net_http_request
where
 url = 'https://httpbin.org/basic-auth/testuser/testpass'
 and user_credentials = 'testuser:testpass';

Result:

url                                              | response_status_code | response_body
-------------------------------------------------|---------------------|----------------------------------
https://httpbin.org/basic-auth/testuser/testpass | 200                 | {"authenticated": true, "user": "testuser"}

Also tested with invalid credentials:

select
  url,
  response_status_code,
  response_body
from
  net_http_request
where
  url = 'https://httpbin.org/basic-auth/testuser/testpass'
  and user_credentials = 'wronguser:wrongpass';

Result:

url                                              | response_status_code | response_body
-------------------------------------------------|---------------------|----------------------------------
https://httpbin.org/basic-auth/testuser/testpass | 401                 |

Also tested with combining insecure = true on an internal API server.