CP-309867: Render the PowerShell cmdlet files from mustache templates (xapi-project#7313)

The PowerShell generator builds each cmdlet's C# file with sprintf, with
the fixed parts of the file mixed in among the code that works out the
variable parts. That makes it hard to see what a generated cmdlet looks
like, and hard to change it. It's also the only SDK generator here with
no tests.

This PR moves those fixed parts into mustache templates. The psd1,
destructor, HTTP action and ConvertTo-XenRef files already work this
way. It also adds golden-file tests for the new templates.

It's a refactor: the generated SDK doesn't change. A full generator run
from this branch is byte-identical to one from master: 413 files,
including all 402 cmdlet .cs files.

Testing

- The generated SDK is byte-identical to master's, checked by diffing a
full generator run from each.
- dune build @fmt, the sdk-gen build, dune test ocaml/sdk-gen/powershell
(7 tests) and ./quality-gate.sh pass on both commits.

Author: daniellamping

ocaml/sdk-gen/powershell/dune

@@ -1,8 +1,18 @@
 (executable
  (modes exe)
  (name gen_powershell_binding)
+ (modules gen_powershell_binding common_functions)
  (libraries astring CommonFunctions mustache xapi-datamodel))
 
+(test
+ (name test_gen_powershell)
+ (package xapi-sdk)
+ (modules test_gen_powershell)
+ (libraries CommonFunctions alcotest fmt mustache xapi-test-utils)
+ (deps
+  (source_tree test_data)
+  (source_tree templates)))
+
 (rule
  (alias generate)
  (targets
@@ -20,7 +30,7 @@
    (bash "rm autogen-out/.gitignore")
    (bash "%{sh} autogen-out/"))))
 
-(data_only_dirs templates)
+(data_only_dirs test_data templates)
 
 (install
  (package xapi-sdk)