feat: Enable flexible HTTP outcalls and (optionally) pay-as-you-go pricing

[eichhorl]

Flipping the FLEXIBLE_HTTP_REQUESTS_FEATURE flag does multiple things:

1. Allows fully- and non-replicated outcalls to select the "pay-as-you-go" pricing version when making the request
2. Allows flexible outcalls (with pay-as-you-go pricing by default) to be made on normal paying subnets
3. Switches flexible outcalls on free and system subnets from legacy pricing to pay-as-you-go pricing (they continue to be free just like before).

Draft spec PR here: dfinity/developer-docs#254

In progress system tests PR: #11327

[zeropath-ai]

✅ No security or compliance issues detected. Reviewed everything up to f0b673e.

Security Overview

• 🔎 Scanned files: 5 changed file(s)
• 🔗 Scan Link: https://zeropath.com/app/repositories/f5e9d2bb-29a4-4b64-902a-391d65fd8b2d?scanId=ff577731-96c1-4457-a545-96467d17f181&codeScanTypes=PrScan&tab=issues

Detected Code Changes

Change Type	Relevant files
Enhancement	► rs/config/src/execution_environment.rs     Enable flexible HTTP requests feature flag ► rs/execution_environment/src/execution_environment/tests.rs     Adapt tests to pay-as-you-go pricing for flexible HTTP requests ► rs/test_utilities/execution_environment/src/lib.rs     Add helper to disable flexible HTTP requests ► rs/tests/networking/canister_http_correctness_test.rs     Adjust imports for new pricing/testing expectations ► rs/execution_environment/src/execution_environment/tests.rs     Update tests to reflect new pricing behavior for flexible HTTP requests ► rs/test_utilities/execution_environment/src/lib.rs     Add with_flexible_http_requests_disabled helper
Bug Fix	► rs/execution_environment/src/execution_environment/tests.rs     Update expectations to PricingVersion::PayAsYouGo in flexible HTTP request tests ► rs/execution_environment/src/execution_environment/tests.rs     Change legacy pricing references to pay-as-you-go where applicable
Refactor	► packages/pocket-ic/tests/tests.rs     Refactor test_canister_http_availability_and_pricing to use unified pricing and simplified loop variables