[opentitanlib] Adds session proxy communication protocol

Introduces a set of serializable struct/enum as implementation of
communication protocol used between 'Proxy Transport Instance' and
opentitansession process.

This protocol is part of issues: lowRISC#10889, lowRISC#10217

Signed-off-by: Michał Mazurek <maz@semihalf.com>
Signed-off-by: Jes B. Klinke <jbk@chromium.org>
diff --git a/sw/host/opentitanlib/BUILD b/sw/host/opentitanlib/BUILD
index fe8df10..20586eb 100644
--- a/sw/host/opentitanlib/BUILD
+++ b/sw/host/opentitanlib/BUILD
@@ -43,6 +43,8 @@
         "src/spiflash/flash.rs",
         "src/spiflash/mod.rs",
         "src/spiflash/sfdp.rs",
+        "src/proxy/mod.rs",
+        "src/proxy/protocol.rs",
         "src/transport/common/mod.rs",
         "src/transport/common/uart.rs",
         "src/transport/cw310/gpio.rs",