refactor(nano): improve custom imports

[glevco]

Depends on #1347

Motivation

This PR provides a few improvements in how the custom __import__ function is provided for blueprints. Currently, it uses the actual builtins.__import__ function internally, but this is not necessary, as we can simply return the allowed and pre-imported objects directly. This adds a new security layer as now it's impossible to call the builtin function, even indirectly, from blueprints.

It also moves the ALLOWED_IMPORTS objects to its own file, to improve organization, and it will be easier to implement custom shells for the allowed objects in a future PR, to provide immutability.

Acceptance Criteria

• Move ALLOWED_IMPORTS to its own file, and provide the pre-imported objects in it.
• Update custom builtins to return the pre-imported objects directly, instead of using the builtins.__import__ function itself.
• Update exposed properties test to not skip the typing module — it works, and finds more exposed cases.

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.71%. Comparing base (48ac10a) to head (aca527d).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1348      +/-   ##
==========================================
+ Coverage   85.69%   85.71%   +0.01%     
==========================================
  Files         425      426       +1     
  Lines       32259    32270      +11     
  Branches     5025     5025              
==========================================
+ Hits        27645    27660      +15     
+ Misses       3602     3600       -2     
+ Partials     1012     1010       -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

The base branch was changed.

[github-actions]

Bencher Report

Branch	refactor/nano/imports
Testbed	ubuntu-22.04

Click to view all benchmark results

Benchmark	Latency	Benchmark Result minutes (m) (Result Δ%)	Lower Boundary minutes (m) (Limit %)	Upper Boundary minutes (m) (Limit %)
sync-v2 (up to 20000 blocks)	📈 view plot 🚷 view threshold	1.71 m (+3.78%) Baseline: 1.64 m	1.48 m (86.72%)	1.81 m (94.34%)

🐰 View full continuous benchmarking report in Bencher