port RuleBasedSampler to opentelemetry-php-contrib

[brettmc]

A proof-of-concept sampler, https://github.com/Nevay/otel-sdk-contrib-sampler was created to demonstrate a rules-based sampler, as an improvement over the sampler proposed in #1208 .
A RuleBasedSampler is mentioned in open-telemetry/oteps#240

Since a Rule-based sampler is not part of the otel specification, it should initially live in opentelemetry-php-contrib repo. It won't be initially supported to enable/configure via environment variables, but it can be configured via code. If the above OTEP is accepted and implemented, then we can use that mechanism to automatically enable/configure the sampler.

If it can be implemented without adding external dependencies (eg symfony/config) that will help if it ever becomes a core component (since we try to minimize dependencies, and code to interfaces).