Add http_proxy configuration option to DNSCrypt-Proxy plugin GUI

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Is your feature request related to a problem? Please describe.
In isolated/air-gapped network environments without direct internet access, DNS traffic needs to be tunneled through an HTTP proxy. The DNSCrypt-Proxy plugin supports the http_proxy parameter natively, but it cannot be configured through the OPNsense GUI. The only workaround is to manually add the parameter to the Jinja2 template /usr/local/opnsense/service/templates/OPNsense/Dnscryptproxy/dnscrypt-proxy.toml, which gets overwritten on every plugin update and must be reapplied each time.

Describe the solution you'd like
I would like an input field in Services → DNSCrypt-Proxy → Configuration analogous to the existing SOCKS5 proxy field, which adds the http_proxy parameter to dnscrypt-proxy.toml.

Describe alternatives you've considered
Manually adding the http_proxy entry directly to the Jinja2 template after the SOCKS5 proxy block:

http_proxy = 'http://<proxy-host>:<port>'

This works but is not persistent across plugin updates.

Additional context
The http_proxy parameter is documented in the official dnscrypt-proxy configuration reference:
https://github.com/DNSCrypt/dnscrypt-proxy/wiki/Configuration

This feature would benefit users in corporate environments where internet access is only available through an HTTP forwarding proxy chain (e.g. Fortigate → Squid → Internet).

Start at Services → DNSCrypt-Proxy → Configuration and find the existing SOCKS5 proxy field, then inspect /usr/local/opnsense/service/templates/OPNsense/Dnscryptproxy/dnscrypt-proxy.toml and its SOCKS5 proxy block. Add the corresponding HTTP proxy configuration so it persists through plugin updates, and verify that the generated dnscrypt-proxy.toml contains the configured http_proxy value.