Provider-agnostic timeouts

[ajcvickers]

The idea here is to have a core-level API to set a "database access timeout". Every provider would then use the value set to configure appropriate timeouts in the lower-level APIs that the provider uses. For example, a relational provider would use this to set both command and connection timeouts.

Any call to a more provider-specific API would override anything set here.

This has two advantages:

• It avoids confusion caused by multiple, subtly different timeouts--e.g. command verses connection
• It allows the in-memory database to either ignore or implement timeouts as appropriate without being a relational provider--see Exception when trying to use InMemoryDatabase for unit testing #6080

[sputier]

As a first step into the EF Core source, I'm trying to think about a way to solve this issue. So far, I thought about adding a DbContextOptionsBuilder.SetDatabaseAccessTimeout method, which would set a CoreOptionsExtension value.
Could this be a good start to handle this issue ?

[AndriySvyryd]

@sputier That sounds reasonable, you can also open a WIP/draft PR if you aren't sure about what needs to be done next. Don't forget to add tests for different providers.