[feature] Allow loading TLS certs from disk (#1586)

Currently, GtS only supports using the built-in LE client directly for
TLS. However, admins may still want to use GtS directly (so without a
reverse proxy) but with certificates provided through some other
mechanism. They may have some centralised way of provisioning these
things themselves, or simply prefer to use LE but with a different
challenge like DNS-01 which is not supported by autocert.

This adds support for loading a public/private keypair from disk instead
of using LE and reconfigures the server to use a TLS listener if we
succeed in doing so.

Additionally, being able to load TLS keypair from disk opens up the path
to using a custom CA for testing purposes avoinding the need for a
constellation of containers and something like Pebble or Step CA to
provide LE APIs.

internal/config/helpers.gen.go

@@ -1524,6 +1524,56 @@ func GetLetsEncryptEmailAddress() string { return global.GetLetsEncryptEmailAddr
 // SetLetsEncryptEmailAddress safely sets the value for global configuration 'LetsEncryptEmailAddress' field
 func SetLetsEncryptEmailAddress(v string) { global.SetLetsEncryptEmailAddress(v) }
 
+// GetTLSCertificateChain safely fetches the Configuration value for state's 'TLSCertificateChain' field
+func (st *ConfigState) GetTLSCertificateChain() (v string) {
+	st.mutex.Lock()
+	v = st.config.TLSCertificateChain
+	st.mutex.Unlock()
+	return
+}
+
+// SetTLSCertificateChain safely sets the Configuration value for state's 'TLSCertificateChain' field
+func (st *ConfigState) SetTLSCertificateChain(v string) {
+	st.mutex.Lock()
+	defer st.mutex.Unlock()
+	st.config.TLSCertificateChain = v
+	st.reloadToViper()
+}
+
+// TLSCertificateChainFlag returns the flag name for the 'TLSCertificateChain' field
+func TLSCertificateChainFlag() string { return "tls-certificate-chain" }
+
+// GetTLSCertificateChain safely fetches the value for global configuration 'TLSCertificateChain' field
+func GetTLSCertificateChain() string { return global.GetTLSCertificateChain() }
+
+// SetTLSCertificateChain safely sets the value for global configuration 'TLSCertificateChain' field
+func SetTLSCertificateChain(v string) { global.SetTLSCertificateChain(v) }
+
+// GetTLSCertificateKey safely fetches the Configuration value for state's 'TLSCertificateKey' field
+func (st *ConfigState) GetTLSCertificateKey() (v string) {
+	st.mutex.Lock()
+	v = st.config.TLSCertificateKey
+	st.mutex.Unlock()
+	return
+}
+
+// SetTLSCertificateKey safely sets the Configuration value for state's 'TLSCertificateKey' field
+func (st *ConfigState) SetTLSCertificateKey(v string) {
+	st.mutex.Lock()
+	defer st.mutex.Unlock()
+	st.config.TLSCertificateKey = v
+	st.reloadToViper()
+}
+
+// TLSCertificateKeyFlag returns the flag name for the 'TLSCertificateKey' field
+func TLSCertificateKeyFlag() string { return "tls-certificate-key" }
+
+// GetTLSCertificateKey safely fetches the value for global configuration 'TLSCertificateKey' field
+func GetTLSCertificateKey() string { return global.GetTLSCertificateKey() }
+
+// SetTLSCertificateKey safely sets the value for global configuration 'TLSCertificateKey' field
+func SetTLSCertificateKey(v string) { global.SetTLSCertificateKey(v) }
+
 // GetOIDCEnabled safely fetches the Configuration value for state's 'OIDCEnabled' field
 func (st *ConfigState) GetOIDCEnabled() (v bool) {
 	st.mutex.Lock()