Spike on fluent ConnectionFactory configuration API

[acogoluegnes]

The ConnectionFactory class has dozens of parameters. It can be overwhelming to configure, especially
for tricky topics like TLS where the parameters are among the other dozens, without clear way to find them.

This commit introduces an API to simplify the configuration of ConnectionFactory. It is fluent, uses modern API (e.g. Duration for timeout), and groups common settings in sub-API.

The configuration API will be introduced in 5.x, marked as experimental, and refined in minor releases.

The traditional setter-based API will be marked deprecated 6.x and removed in 7.x.

Benefits of the new configuration API:

• fluent, the method calls can be chained and formatted in a logical way (it does not have to be 1 line = 1 parameter).
• the different timeout settings use the Duration type, instead of int. It is no longer necessary to know the unit (seconds or milliseconds).
• parameters for the same topic are grouped into dedicated configuration API (TLS, NIO, OAuth2, recovery, etc). It makes it much easier to configure those parts, as available settings will show up automatically in the IDE auto-completion, and not among the other dozens of settings.
• more opinionated but easier configuration. OAuth2 is an example: no need to use builder classes with very long names, the refresh service part is also in the #oauth2() sub-configuration, whereas it's a separate setter in ConnectionFactory.

References #608, #1139

There is a demo class to show the differences between the regular and new API.