[dynamic control] add initializing from declarative config

[jackshirazi]

Description:

Sorry for the change set being a little larger than normal, but I thought it was better to combine all the declarative config support changes together, rather than adding them piecemeal. I think it may be easier to understand the whole than the parts

This contains

• a DeclarativeConfigurationCustomizerProvider that reads top-level telemetry_policy/development from declarative config and routes it into PolicyInit
• a declarative ComponentProvider for telemetry_policy/development to add the delagating sampler - initially for the rate sampling policy, but will later make more generic (includes a TODO for that)
• Changes to PolicyInit to support the declarative config and also make it a one-shot execution so that a later/repeated/accidental auto-config pass does not accidentally re-use stale declarative config

Existing Issue(s):

part of #2868

Testing:

Unit tests added, plus a manual end-to-end test using declarative config was performed

Documentation:

TBD see #2868

Outstanding items:

see #2868

[jackshirazi]

@jack-berg I mentioned a while back that the documentation for this type of declarative config addition was minimal, so would love for you to review the implementation here.