DLP: how long held mail waits is a setting

inbuxa:DlpSettings (singleton, urn:inbuxa:jmap): keepHeldDays, 1 to 90,
7 by default (settled answer 5 made it a setting). sysDlpPolicyGet reads
it, sysDlpPolicyUpdate changes it, server-level, audited by the request
layer. Each held message keeps the days it was given, and the sender's
notices say that number. Privacy catalog entry; spec §2.6 updated.

mail_rules_tests: 7 by default, 0 refused, 3 set and a message held
afterwards expires 3 days after it was held, the expiry notice says 3.

crates/jmap-proto/src/request/parser.rs

@@ -176,6 +176,13 @@ impl<'de> Visitor<'de> for CallVisitor {
                     return Err(de::Error::invalid_length(1, &self));
                 }
             },
+            (MethodFunction::Get, MethodObject::DlpSettings) => match seq.next_element() {
+                Ok(Some(value)) => RequestMethod::Get(GetRequestMethod::DlpSettings(value)),
+                Err(err) => RequestMethod::invalid(err),
+                Ok(None) => {
+                    return Err(de::Error::invalid_length(1, &self));
+                }
+            },
             (MethodFunction::Get, MethodObject::DataInventory) => match seq.next_element() {
                 Ok(Some(value)) => RequestMethod::Get(GetRequestMethod::DataInventory(value)),
                 Err(err) => RequestMethod::invalid(err),
@@ -378,6 +385,13 @@ impl<'de> Visitor<'de> for CallVisitor {
                     return Err(de::Error::invalid_length(1, &self));
                 }
             },
+            (MethodFunction::Set, MethodObject::DlpSettings) => match seq.next_element() {
+                Ok(Some(value)) => RequestMethod::Set(SetRequestMethod::DlpSettings(value)),
+                Err(err) => RequestMethod::invalid(err),
+                Ok(None) => {
+                    return Err(de::Error::invalid_length(1, &self));
+                }
+            },
             (MethodFunction::Set, MethodObject::Explanation) => match seq.next_element() {
                 Ok(Some(value)) => RequestMethod::Set(SetRequestMethod::Explanation(value)),
                 Err(err) => RequestMethod::invalid(err),