Add autoclose option to close backend when session closes

[JWCook]

Closes #153. Also see #146

This is enabled by default only for the SQLite backend.

[codecov]

Codecov Report

Merging #162 (4f4a3f7) into main (f862e6d) will increase coverage by 0.39%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #162      +/-   ##
==========================================
+ Coverage   94.77%   95.17%   +0.39%     
==========================================
  Files          11       11              
  Lines        1015     1016       +1     
  Branches      181      181              
==========================================
+ Hits          962      967       +5     
+ Misses         38       34       -4     
  Partials       15       15              

Impacted Files	Coverage Δ
aiohttp_client_cache/backends/redis.py	98.24% <ø> (+3.24%)	⬆️
aiohttp_client_cache/backends/base.py	96.72% <100.00%> (+1.19%)	⬆️
aiohttp_client_cache/backends/sqlite.py	99.09% <100.00%> (-0.01%)	⬇️
aiohttp_client_cache/session.py	91.11% <100.00%> (+0.20%)	⬆️
aiohttp_client_cache/signatures.py	76.74% <100.00%> (ø)