expiry: different options

[sckott]

right now the only way expire time works is across all stubbed/cached requests, that is, if an expire time is set, then on the next http request using webmiddens, all stubs are deleted that are expired

i imagine more fine grained control may be useful:

• set expire time for all http requests matching a given http request pattern - i guess via any of the webmockr options, e.g., a url pattern, or a header pattern, or a body pattern
• a user of a package foobar may want to set different expire times for each function of a package they're using, e.g. 10 sec for fxn1(), 1 minute for fxn2(), and 1 week for fxn3()
• what else?