[Feature request] Add support for self-hosted Gitlab(CE) or GitHub Enterprise

Feature request: add support for the **self-hosted** version of Gitlab (CE).

Currently the Amplify console has support for Gitlab.com, but not for the self-hosted variant.
At this moment it is unfortunately a dealbreaker why we do not yet use Amplify Console for all projects/sites.
As a workaround we now use the S3 and CodeCommit (by using the mirror repository function of Gitlab).

**2025-01-28, Update from Amplify Team:**

> [swaminator](https://github.com/aws-amplify/amplify-hosting/issues/14#issuecomment-2617342798): Hi folks, just to give you an update this item has been prioritized for 2025. Stay tuned.

**2025-02-19, Question from Amplify Team:**
> [swaminator](https://github.com/aws-amplify/amplify-hosting/issues/14#issuecomment-2669272890): Are there preferences folks have that are outside self-hosted GitLab or GitHub enterprise? Any other providers?

**2025-03-03, Question from Amplify Team:**
> [mauerbac](https://github.com/aws-amplify/amplify-hosting/issues/14#issuecomment-2776709846): Matt from the Amplify Hosting team is asking about the hosting setup for GitLab/GitHub instances, specifically whether they are private, if VPC Peering or AWS PrivateLink support is needed, and how access should be configured (e.g., bastion host, IP restrictions, or public webhooks).

Start by comparing the existing GitLab.com support with the self-hosted GitLab CE and GitHub Enterprise cases described here, then review the hosting setup and access requirements raised in the comments. Done means Amplify Console supports the requested self-hosted providers with the required connectivity and access configuration agreed and verified.

Written by the indexing model from the issue text.