install_report: add pip_version

Author: sbidoul

docs/html/reference/installation-report.md

@@ -7,6 +7,7 @@ it did install (or what it would have installed, if used with the `--dry-run` op
 
 The report is a JSON object with the following properties:
 
+- `pip_version`: a string with the version of pip used to produce the report.
 - `install`: an object where the properties are the canonicalized names of the
   distribution packages (to be) installed and the values are of type
   `InstallationReportItem` (see below).
@@ -51,6 +52,7 @@ will produce an output similar to this (metadata abriged for clarity):
 
 ```json
 {
+  "pip_version": "22.2",
   "install": {
     "pydantic": {
       "download_info": {

src/pip/_internal/models/installation_report.py

@@ -3,6 +3,7 @@
 from pip._vendor.packaging.markers import default_environment
 from pip._vendor.packaging.utils import canonicalize_name
 
+from pip import __version__
 from pip._internal.req.req_install import InstallRequirement
 
 
@@ -38,6 +39,7 @@ def _install_req_to_dict(cls, ireq: InstallRequirement) -> Dict[str, Any]:
 
     def to_dict(self) -> Dict[str, Any]:
         return {
+            "pip_version": __version__,
             "install": {
                 canonicalize_name(ireq.metadata["Name"]): self._install_req_to_dict(
                     ireq