[attestation] Rename the client VM attestation API

As agreed in the doc go/pvm-remote-attestation.

Bug: 303807447
Test: m com.android.virt
Test: run ServiceVmClientApp
Change-Id: Ic954aa60d69e42773c04230af9319e660352d6a9
diff --git a/virtualizationservice/src/rkpvm.rs b/virtualizationservice/src/rkpvm.rs
index d6e87eb..443b280 100644
--- a/virtualizationservice/src/rkpvm.rs
+++ b/virtualizationservice/src/rkpvm.rs
@@ -21,7 +21,7 @@
 use service_vm_comm::{GenerateCertificateRequestParams, Request, Response};
 use service_vm_manager::ServiceVm;
 
-pub(crate) fn request_certificate(csr: &[u8]) -> Result<Vec<u8>> {
+pub(crate) fn request_attestation(csr: &[u8]) -> Result<Vec<u8>> {
     let mut vm = ServiceVm::start()?;
 
     // TODO(b/271275206): Send the correct request type with client VM's