Add install_cache function

[karpetrosyan]

This pull request simply adds the install_cache function, which mocks the httpx.Client and httpx.AsyncClient and adds HTTP caching on top.

Example:

import hishel
import httpx

hishel.install_cache()

client = httpx.Client()
client.get("https://hishel.com")
client.get("https://hishel.com")  # takes from the cache

[codecov]

Codecov Report

Merging #95 (b720a61) into master (2af3363) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #95   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           27        27           
  Lines         1665      1666    +1     
=========================================
+ Hits          1665      1666    +1     

Files	Coverage Δ
hishel/__init__.py	100.00% <100.00%> (ø)

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