improved ForcedSampling support

[willarmiros]

Issue #, if available:
#230

Description of changes:
Adds check for ForcedSamplingSupport to beginSegmentWithSampling. Also makes ForcedSampling a configurable property for the existing Centralized and Localized sampling strategies via new constructors. I did not add them to the All or No strategies since forced sampling doesn't make sense in either case, they should be all or nothing, respectively.

I used constructor parameters to keep the new field final. One alternative would be to implement builder classes for the strategies (4 new builder classes seems overkill tho) or just one sampling strategy builder that works like:

SamplingStrategy strategy = SamplingStrategyBuilder
    .withStrategy("Centralized")  // Or withCentralizedSampling() ?
    .withRuleLocation(someUrl)  // These are no-ops for All and None strategies
    .withForcedSampling()
    .build();

Thoughts @anuraaga ?

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[srprash]

From what I understand of ForcedSampling is that it can be used to force sample a segment which is un-sampled when created. Or is it only to keep the segment in context even though it is un-sampled?

[willarmiros]

Forced sampling forces the creation of a regular instead of no-op segment when not sampled, which also places it in context. Then forceSamplingOfCurrentSegment can be used in conjunction to actually switch an unsampled segment to sampled.

[srprash]

This is addressed now right? :)

[willarmiros]

Fair enough haha

[anuraaga]

I think the constructor parameters are OK - the builder seems nice if we were to also hide the classes, but that would be breaking, so maybe a future cleanup