Consistent delegating sampler, alternative to ParentBased

[jmacd]

Part of #3602.

Changes

Adds Consistent sampler, a delegating consistent sampler to replace ParentBased.
Adds alternate Sampler API option for efficient sampler delegation and threshold management
Changes default sampler to Consistent(TraceIdRatio(1)).
Has TODO referring to supplemental guidelines on composite samplers, to be added in tracestate-probability-sampling.md after #4162 and #4166 gave merged. These rules will spell out how to form conditional forms of delegating samplers correctly, including the logic behind the Consistent sampler decision.

This absolutely needs a prototype. As seen in open-telemetry/opentelemetry-go#5645, there are immediately performance concerns when we implement consistent sampling decisions without the sampler API optimization described in this PR.

• Related issues #
• Related OTEP(s) #
• Links to the prototypes (when adding or changing features)
• CHANGELOG.md file updated for non-trivial changes
• spec-compliance-matrix.md updated if necessary

[jmacd]

Sampling SIG would prefer to see the conditional sampler introduced in OTEP 250 specified first, after which the Consistent one proposed here becomes a composition of two predicates (root? local?).

Users of the existing ParentBased sampler, which allows composition to extend attributes and trace state will need a way to do that. @PeterF778's prototype used a "Marker" sampler for that purpose. I will create a new draft.