[dynamic control] Add file reader with line-by-line policy provider

[jackshirazi]

Per #2546

A basic policy provider which reads a file line-by-line (supporting JSON format and ley=value format) and returns policies

A full working implementation of the open-telemetry/opentelemetry-specification#4738 is here in this branch

[Copilot]

Pull request overview

Adds a new PolicyProvider SPI and an initial file-backed implementation that reads one policy per line (JSON or key=value) to produce TelemetryPolicy instances.

Changes:

• Introduces PolicyProvider interface with polling (fetchPolicies) and optional push (startWatching) update mechanism.
• Adds LinePerPolicyFileProvider to load policies from a local file with basic parsing and validator dispatch.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 6 comments.

File	Description
dynamic-control/src/main/java/io/opentelemetry/contrib/dynamic/policy/PolicyProvider.java	Defines the provider contract for fetching and optionally watching policy updates.
dynamic-control/src/main/java/io/opentelemetry/contrib/dynamic/policy/LinePerPolicyFileProvider.java	Implements file-based policy loading and validation dispatch for JSON / alias lines.

[jaydeluca]

lgtm.

cc @LikeTheSalad for final review