Make `refresh_lead_time` customizeable

[MaeIsBad]

Environment

• ExAws from the main branch(8020fa1c0a7221c3931a91ec8089e52c868667c0)

Current behavior

The ExAws.Config.AuthCache module configures the refresh_lead_time as an attribute, preventing users from modifying it

Expected behavior

Users can configure the token refresh leeway

Additional Notes

An option like this is exposed in the official aws rust sdk as buffer_time

https://docs.rs/aws-config/latest/aws_config/identity/struct.LazyCacheBuilder.html#method.buffer_time