feat: add webview for security findings

Implements #715 (closed)

Description

Add a new WebView for Vulnerability findings and display the the following information for every finding:

1. title
2. description
3. severity

Screenshot

Steps to reproduce

When member of the project

1. Ask to be added as a developer or maintainer to https://gitlab.com/fernando-c/test-vs-code-security-reports/

2. Clone the project with vulnerability findings fernando-c/test-vs-code-security-reports!1 (closed)

3. Enable the feature flag in the cloned project in .vscode/settings.json

{
    "gitlab.featureFlags.securityScansFlag": true
}

3. git checkout master-patch-211c

4. Check for security findings in the current branch sidebar for the gitlab extension

5. Every security findings information should displayed correctly in the webview

When not a member of the project

1. Clone the project with vulnerability findings https://gitlab.com/svedova/gitlab-mr-widgets-demo NOTE this project is different from the project above

2. Enable the feature flag in the cloned project in .vscode/settings.json

{
    "gitlab.featureFlags.securityScansFlag": true
}

3. git checkout main-patch-2326 NOTE this is different than the branch in the steps above

4. Check for security findings in the current branch sidebar for the gitlab extension

5. You should see No Scans Found