Feature: TransactionContext, InstructionContext and BorrowedAccount (#21706)

* Adds TransactionContext, InstructionContext and BorrowedAccount.

* Redirects the usage of accounts in InvokeContext through TransactionContext.
Also use the types declared in transaction_context.rs everywhere.

* Adjusts all affected tests.

sdk/src/lib.rs

@@ -46,6 +46,7 @@ pub mod signer;
 pub mod system_transaction;
 pub mod timing;
 pub mod transaction;
+pub mod transaction_context;
 pub mod transport;
 pub mod wasm;