Support custom / internal ACME CA using HTTP-01 challenge

Is your feature request related to a problem? Please describe.
Nginx Proxy Manager only supports Let’s Encrypt as an ACME provider and does not allow configuring a custom ACME directory URL.
This prevents using NPM with an internal or private ACME CA, even when the CA supports the standard HTTP-01 challenge.

Describe the solution you'd like
Add support for a custom / internal ACME CA with:

• Ability to configure a custom ACME directory URL
• Ability to trust a custom root/intermediate CA
• Support for HTTP-01 challenge for certificate issuance and renewal

Describe alternatives you've considered

• Generating certificates externally using an ACME client and importing them as “Custom Certificates” in NPM
• Using a different reverse proxy that supports custom ACME CAs

Additional context
This would enable fully automated certificate management in homelab and private environments without relying on Let’s Encrypt.

The issue names no files, tests, or entry points. Start by locating the existing Let's Encrypt ACME configuration and HTTP-01 issuance and renewal flow, then trace how certificate authority trust is configured. Done means a custom directory URL and custom root or intermediate CA can support automated HTTP-01 issuance and renewal.