[Feat] browser should support custom base url for bing search

[Mai0313]

Why are these changes needed?

The browser utils which is called by WebSurferAgent should support custom base_url
This base_url has been named as bing_base_url since I wanna remain the same format/naming rule with bing_api_key.

This change has been tested using my own url; bing_base_url is optional field.

A simple pre-view of how to use is:

browser_config={
    "viewport_size": 4096,
    "bing_api_key": bing_api_key,  # the api from my company only accepts api_key in headers; but I decided to keep this as the same for normal usage.
    "bing_base_url": "https://mlop-azure-gateway.mediatek.inc/v7.0/search",
    "request_kwargs": {
        "verify": False,
        "headers": {
            "X-User-Id": "mtk30765",
            "api-key": bing_api_key
        }
    }
},

Related issue number

#2320

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 36.55%. Comparing base (4a44093) to head (5034f08).
Report is 1 commits behind head on main.

Files	Patch %	Lines
autogen/browser_utils.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2321      +/-   ##
==========================================
- Coverage   38.14%   36.55%   -1.59%     
==========================================
  Files          78       76       -2     
  Lines        7865     7859       -6     
  Branches     1683     1684       +1     
==========================================
- Hits         3000     2873     -127     
- Misses       4615     4742     +127     
+ Partials      250      244       -6     

Flag	Coverage Δ
unittests	36.55% <0.00%> (-1.58%)	⬇️

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[sonichi]

@jackgerrits is this PR ready to merge?