refactor: simplify webhook deployment api(breaking)

[klesh]

Summary

Currently, the Webhook Deployment API supports 2 ways of creating a deployment.

1. Creating a Deployment that deploys a single repo

POST /plugins/webhook/connections/1/deployments
{
  "pipeline_id": "string",
  "repo_id": "string",
  "repo_url": "string",
  "commit_sha": "string"
   ....
}

2. Creating a Deployment that deploys multiple repos

POST /plugins/webhook/connections/1/deployments
{
  "pipeline_id": "string",
  "deployment_commits": [
    {
      "repo_id": "string",
      "repo_url": "string",
      "commit_sha": "string"
    },
    {
      "repo_id": "string",
      "repo_url": "string",
      "commit_sha": "string"
    }
  ]
}

This might seem useful for different cases, but it also introduces ambiguity: For those who want to post multiple repos, they often get confused about the repo_id, repo_url and commit_sha in the root node, including our core maintainer.

So, we concluded it would be simpler for both users and maintainers if we supported only one format.

This PR would introduce the following breaking-changes:

1. Removing the support of the single repo form while keeping the latter which could support both scenarios.
2. Changing the field name spelling from snake_case to camelCase to align with other API designs.

The final API request would be:

POST /plugins/webhook/connections/1/deployments

{
  "id": "deploy pipeline12 to somewhere",
  "pipelineId": "pipeline12",
  "startedTime": "2024-01-19T12:00:00+00:00",
  "endedTime": "2024-01-20T12:00:00+00:00",
  "result": "SUCCESS",
  "deploymentCommits":[
    {
      "repoId": "optional",
      "startedTime": "2024-01-19T12:00:00+00:00",
      "endedTime": "2024-01-20T12:00:00+00:00",
      "repoUrl": "https://example.com",
      "refName": "release-v0.19",
      "commitSha": "sha-v0.19",
      "commitMsg": "optional"
    }
  ]
}