use fake-useragent for UserAgent

[w-e-w]

Description

@SpenserCai
as opposed to hard coding a relatively outdated user agent

stable-diffusion-webui/modules/api/api.py

Line 62 in 4760c3c

response = requests.get(encoding, timeout=30, headers={'user-agent':'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/62.0.3202.94 Safari/537.36'})

chrome 62.0.3202 is from 2017-10-17, it screams bot

we can use a library like fake-useragent https://pypi.org/project/fake-useragent

kind of related PR #12698

a test script

from fake_useragent import FakeUserAgent
ua = user_agent = FakeUserAgent(min_percentage=1.0).random
print(ua)

another method would be to just expose the user method make it editable

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• My code follows the style guidelines
• My code passes tests

[AUTOMATIC1111]

I added an option to let user specify the user agent if he needs one, and removed the fake one as default.