Allow using custom executors in SaslAuthenticationFactory

The SaslAuthenticationFactory.Builder uses a hardcoded ScheduledExecutor which adds an implicit dependency on jboss-threads. It would be nice if the builder allowed a user-provided executor.

Discussion from Zulip:

Tristan Tarrant: @Darran Lofthouse I have a request: in https://github.com/wildfly-security/wildfly-elytron/blob/1.x/auth/server/sasl/src/main/java/org/wildfly/security/auth/server/sasl/SaslAuthenticationFactory.java#L143 would it be possible to pass in a custom scheduled executor and possibly forgo the dependency on jboss-threads. I have my own executors/pool and I'd rather use those instead.

Darran Lofthouse: @Farah Juma FYI ^^^

@Tristan Tarrant Sounds reasonable, we may need to look at how the SecurityDomain loads the class ScheduledExecutorServiceProvider as I suspect you don't want the JBossThreads impls loading at all.

Farah Juma: +1, sounds ok

Farah Juma: @Tristan Tarrant Please create an ELY JIRA issue to track this

Darran Lofthouse: I suspect in other cases it will be beneficial to drop the JBoss Threads dependency

David Lloyd: probably in most cases; it just thinly wraps the standard ScheduledThreadPoolExecutor

Darran Lofthouse: So maybe better then for Elytron to use the standard ScheduledThreadPoolExector add an override as Tristan suggests and then if anyone really wants JBossThreads they can add it in using the override.