[Config Registry] 1/5 Add gitlab step and json configuration file and yaml for doc (#7548)

## Context

Part of **Configuration Inversion (Step 1)** - Stack progress:
1. **→ [#7548](#7548) -
Add GitLab step and JSON configuration file (this PR)**
2. [#7688](#7688) -
Reorganize keys, split config keys and platform keys
3. [#7698](#7698) -
Generate ConfigurationKeys with source generator
4. [#7689](#7689) -
Aliases handling and analyzers
5. [#7697](#7697) -
Replace manual ConfigurationKeys by generated ones in the whole solution



## Summary

Adds centralized configuration registry (`supported-configurations.json`
+
[supported-configurations-docs.yaml](cci:7://file:///Users/anna.yafi/go/src/github.com/DataDog/dd-trace-dotnet3/tracer/src/Datadog.Trace/Configuration/supported-configurations-docs.yaml:0:0-0:0))
containing ~280 configuration keys with automated GitLab CI validation.

## Changes

**Configuration Files:**
- `supported-configurations.json` - Machine-readable registry of all
configuration keys with versions, aliases, and deprecations
-
[supported-configurations-docs.yaml](cci:7://file:///Users/anna.yafi/go/src/github.com/DataDog/dd-trace-dotnet3/tracer/src/Datadog.Trace/Configuration/supported-configurations-docs.yaml:0:0-0:0)
- Human-readable documentation for each configuration key

**CI/CD:**
- Added `validate_supported_configurations_local_file` step to
`.gitlab-ci.yml`
- Updated `.gitlab/one-pipeline.locked.yaml` to reference validation
template

**File Structure:**
```json
{
  "supportedConfigurations": { "DD_TRACE_ENABLED": { "version": ["A"] }, ... },
  "aliases": { "DD_AGENT_HOST": ["DD_TRACE_AGENT_HOSTNAME", ...], ... },
  "deprecations": { "DD_TRACE_ANALYTICS_ENABLED": "Deprecated - ...", ... }
}
```

## Motivation

Provides single source of truth for configuration keys to enable:
- Automated documentation validation
- Cross-tracer compatibility checks
- Configuration migration tooling
- Automated doc generation

See: [Configuration Registry Design
Doc](https://docs.google.com/document/d/1VIvOyQPfuQIWBIgNY68A9hzLir0-3eJijTwmpWASO8s/edit?tab=t.0#heading=h.jy2ajbk2b8ba)

## Validation

GitLab CI validates JSON structure, naming conventions, and schema
compliance on every build.

## Commit Details

**`2639cf0` - Initial Configuration Registry**
- Added `supported-configurations.json` with ~280 configuration keys
organized by version (A/B/C)
- Includes `aliases` section mapping primary keys to
deprecated/alternative names
- Includes `deprecations` section with deprecation messages
- Added GitLab CI validation step to ensure file integrity
- Updated one-pipeline lock file to reference validation template

**`70e632e` - Configuration Documentation**
- Added
[supported-configurations-docs.yaml](cci:7://file:///Users/anna.yafi/go/src/github.com/DataDog/dd-trace-dotnet3/tracer/src/Datadog.Trace/Configuration/supported-configurations-docs.yaml:0:0-0:0)
with detailed XML documentation for each configuration key
- Provides human-readable descriptions, default values, and usage
examples
- Includes `<seealso>` references to relevant code classes
- Complements the machine-readable JSON with developer-friendly
documentation
- Enables automated generation of configuration documentation from a
single source

Author: anna-git

.gitlab-ci.yml

@@ -232,3 +232,12 @@ dsm_throughput:
     - if: '$CI_PIPELINE_SOURCE == "schedule" && $BENCHMARK_RUN == "true"'
       when: always
     - when: manual
+
+
+validate_supported_configurations_local_file:
+  stage: build
+  rules:
+    - when: on_success
+  extends: .validate_supported_configurations_local_file
+  variables:
+    LOCAL_JSON_PATH: "tracer/src/Datadog.Trace/Configuration/supported-configurations.json"

.gitlab/one-pipeline.locked.yaml

@@ -1,4 +1,4 @@
 # DO NOT EDIT THIS FILE MANUALLY
 # This file is auto-generated by automation.
 include:
-  - remote: https://gitlab-templates.ddbuild.io/libdatadog/one-pipeline/ca/5ad4e568659a0e385e3cd429b7845ad8e2171cfe0e27ee5b9eeb4cd4b67825f5/one-pipeline.yml
+  - remote: https://gitlab-templates.ddbuild.io/libdatadog/one-pipeline/ca/b8fc1b7f45e49ddd65623f217f03c38def169aff6f2518380e5b415514e4cb81/one-pipeline.yml