Add support for shared context and tagged log entries

[solnic]

This adds support for setting arbitrary context data that's shared across all backends while logging and also tagged log entries:

irb(main):002:0> logger = Dry.Logger(:my_app)
=> 
#<Dry::Logger::Dispatcher:0x0000000106ae9b40
...
irb(main):003:0> logger.context[:component] = "test"
=> "test"
irb(main):004:0> logger.info "hello"
hello component="test"

irb(main):005:0> logger.tagged("red") { logger.warn("oops") }
oops component="test",tag="red"