[Serializer] Streamlined context management

[dkarlovi]

Currently, serializer context is quite simple by default which prevents from doing certain things:

1. setting app-wide settings (for example, allow_extra_attributes=false as in disallow extra field when denormalization #1217)
2. setting resource-wide settings
3. setting request-specific settings (for example, setting per-user / OAuth client timezone for DateTime (de)normalization)

IMO it would make sense to provide a way to build a context, maybe with a new service tag? WDYT?

[soyuka]

What about overriding SerializerContextBuilder?

[dkarlovi]

It gets complex as you want to add resource-specific contexts.

It does make it somewhat possible, but it's not great. @dunglas agreed: #1217 (comment)

[dunglas]

This has been fixed (at least partially) by #3151