Fix code scanning alert no. 2: Incomplete URL substring sanitization

[guruh46]

Fixes https://github.com/guruh46/vscode-gitlens/security/code-scanning/2

To fix the problem, we need to parse the URL and check its host value against a whitelist of allowed hosts. This ensures that the check handles arbitrary subdomain sequences correctly and prevents malicious URLs from bypassing the security check.

1. Parse the URL using the url module to extract the host.
2. Check if the host is in a predefined list of allowed hosts.
3. Update the getHeaders method to use this new check.

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