[TT-10153] Memory Leak fix

[mativm02]

Description

Now we're using a Singleton to generate new HTTP handlers. This will help us to avoid memory leaks due to the multiple creation of http.Handler objects.

Related Issue

https://tyktech.atlassian.net/browse/TT-10153?atlOrigin=eyJpIjoiYmY2YzE3OWNhNjE2NDliMGI3YmQzYTU4YTU0ZDdhMTEiLCJwIjoiaiJ9

Motivation and Context

https://community.tyk.io/t/increasing-memory-usage-after-5-2-upgrade/6731

Test Coverage For This Change

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactoring or add test (improvements in base code or adds test coverage to functionality)
• Documentation updates or improvements.

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[tbuchaillot]

LGTM

[tbuchaillot]

Found a big problem with this approach - we actually need multiple HTTP handler for each API since we are setting different attributes for each handler ( like api_id, org_id, etc).
We need to continue investigating.