[feature] Add hooks for validate and validate_build methods

[uilianries]

Motivation

Hello!

This PR brings a new hooks for validate() and validate_build() methods.

The intention is to help CI services validate a build without touching the recipe, by adding an extra layer of validations. For instance, the CI environment could not build a determined package due to custom settings, or some system library is not available because the O.S. is too old.

Details

Both validate and validate_build don't have a specific command, like export or build, so I used conan create to get a ride and check if is working.

Also, both methods are not defined by default, which means, the hook will only run when one of these methods is defined in the recipe.

Automation

Changelog: Feature: Add hooks pre_validate, post_validate, pre_validate_build and post_validate_build
Docs: TODO

/cc @jcar87

• Refer to the issue that supports this Pull Request.
• If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
• I've read the Contributing guide.
• I've followed the PEP8 style guides for Python code.
• I've opened another PR in the Conan docs repo to the develop branch, documenting this one.

[memsharded]

This can make the batch processing at scale in repos like ConanCenter very slow, as this will be instantiated tons of times. Need to be factored out from here (this might be complicated and require some re-architecture)

cc @AbrilRBS

[uilianries]

I just need to access the Conan App to capture the hook paths, then, instantiate the hook manager. It would be great doing it in run_validate_package_id directly, but I have no access to Conan App. Is there a way to capture the Conan App globally? I also don't like this current approach of forwarding the hook manager til run_validate_package_id

[AbrilRBS]

Note that there's an easy lazy load optimization available for the HookManager that would solve the issue, something like this might be worth looking into:
that would almost solve it, but you still would need to modify the class instance instead of self._hooks to avoid the slowness:

diff --git a/conans/client/hook_manager.py b/conans/client/hook_manager.py
index d213b209d..9eb87bdb3 100644
--- a/conans/client/hook_manager.py
+++ b/conans/client/hook_manager.py
@@ -12,11 +12,18 @@ valid_hook_methods = ["pre_export", "post_export",
 
 
 class HookManager:
+    _hooks = None
 
     def __init__(self, hooks_folder):
         self._hooks_folder = hooks_folder
-        self.hooks = {}
-        self._load_hooks()  # A bit dirty, but avoid breaking tests
+
+    @property
+    def hooks(self):
+        if self._hooks is None:
+            # Have this before the call below, or its accesses to hook will be recursive
+            self._hooks = {}
+            self._load_hooks()
+        return self._hooks
 
     def execute(self, method_name, conanfile):
         assert method_name in valid_hook_methods, \

[AbrilRBS]

But then if you access/modify just HookManager._hooks, you'll have issues with the tests keping states between each call so, pick your poison I guess