[Discussion][Connector-V2] Establish connector certification levels and acceptance test templates

## Background
SeaTunnel has a large and growing connector ecosystem, but users and maintainers still lack a shared way to judge connector maturity.

Over time, this becomes a governance problem:
- users cannot tell which connectors are safe for production use
- reviewers do not have a shared maturity baseline
- connector quantity can grow faster than connector quality

## Problem to solve
We need a practical way to communicate connector maturity and to tie that maturity to repeatable acceptance checks.

## Proposal
Introduce connector certification levels together with acceptance test templates.

Suggested first-delivery goals:
- define a small number of certification levels
- define what each level means
- provide reusable acceptance test templates for each level
- pilot the model on a small set of high-usage connectors

Example level direction for discussion during implementation:
- Basic
- Stable
- Production
- CDC Production

## Scope
Phase 1 should stay intentionally lightweight:
- define the levels
- define the corresponding acceptance template expectations
- run a pilot on a small number of connectors

## Non-goals
- do not force every connector to reach Production immediately
- do not turn the system into a heavy scoring framework
- do not make this a documentation-only badge exercise

## Suggested validation
- validate that trial connectors can be classified consistently
- validate that each level maps to a concrete test template
- validate that reviewers can use the templates to evaluate maturity more quickly and more consistently

## Acceptance criteria
- each certification level has a clear meaning
- each level has at least one reusable acceptance template
- at least a small pilot set of connectors is classified using the new model
- the result is usable by maintainers and users, not only by docs authors

## Are you willing to submit a PR?
- [ ] Yes I am willing to submit a PR!

No files, tests, or entry points are named. Start by reviewing the connector ecosystem and the proposed certification levels, then define lightweight level meanings and reusable acceptance templates; done means a small pilot classifies connectors consistently and maintainers and users can use the results.

Written by the indexing model from the issue text.