feat: add support for MSC4439 PGP key URIs in wk-support

This commit introduces support for MSC4439, Encryption Key URIs
in `.well-known/matrix/support`. ([MSC](https://github.com/matrix-org/matrix-spec-proposals/pull/4439),
[Rendered](https://github.com/thetayloredman/matrix-spec-proposals/blob/msc4439/proposals/4439-support-contact-encryption.md))
via an additional config option.

src/api/client/well_known.rs

@@ -71,6 +71,7 @@ pub(crate) async fn well_known_support(
 
 	let email_address = services.config.well_known.support_email.clone();
 	let matrix_id = services.config.well_known.support_mxid.clone();
+	let pgp_key = services.config.well_known.support_pgp_key.clone();
 
 	// TODO: support defining multiple contacts in the config
 	let mut contacts: Vec<Contact> = vec![];
@@ -88,6 +89,7 @@ pub(crate) async fn well_known_support(
 			role: role_value.clone(),
 			email_address: email_address.clone(),
 			matrix_id: matrix_id.clone(),
+			pgp_key: pgp_key.clone(),
 		});
 	}
 
@@ -104,6 +106,7 @@ pub(crate) async fn well_known_support(
 				role: role_value.clone(),
 				email_address: None,
 				matrix_id: Some(user_id.to_owned()),
+				pgp_key: None,
 			});
 		}
 	}

src/core/config/mod.rs

@@ -2191,6 +2191,13 @@ pub struct WellKnownConfig {
 	/// listed.
 	pub support_mxid: Option<OwnedUserId>,
 
+	/// PGP key URI for server support contacts, to be served as part of the
+	/// MSC1929 server support endpoint. This uses the unstable prefix from
+	/// [MSC4439].
+	/// 
+	/// [MSC4439]: https://github.com/matrix-org/matrix-spec-proposals/pull/4439
+	pub support_pgp_key: Option<String>,
+
 	/// **DEPRECATED**: Use `[global.matrix_rtc].foci` instead.
 	///
 	/// A list of MatrixRTC foci URLs which will be served as part of the