Avoid nesting tokio runtimes

[kate-goldenring]

Signed-off-by: Kate Goldenring kate.goldenring@microsoft.com

What this PR does / why we need it:
OPC UA discovery is currently failing with the error Cannot drop a runtime in a context where blocking is not allowed. This happens when a runtime is dropped from within an asynchronous context as described in #483. This is because for discovery, an OPC UA client is created and then dropped. The client uses an async TCP connection, which is dropped when the client is dropped. There are two nested runtimes, the spawned thread in discover and the TCP connection. To avoid this, the same runtime should be used rather than nesting, which can be done via spawn_blocking -- this is a good discussion.

Special notes for your reviewer:

• closes Akri DEV Helm Chart 0.8.12 - akri-opcua-discovery-daemonset Failure #483
• also updated to latest opcua crate
• also autogenerating certs to avoid errors due to lack of certs

Note: This PR fixes this bug, but a next iteration of this would be do consider creating an OPC UA client once rather than in each discover iteration.

If applicable:

• this PR has an associated PR with documentation in akri-docs
• this PR contains unit tests
• added code adheres to standard Rust formatting (cargo fmt)
• code builds properly (cargo build)
• code is free of common mistakes (cargo clippy)
• all Akri tests succeed (cargo test)
• inline documentation builds (cargo doc)
• version has been updated appropriately (./version.sh)
• all commits pass the DCO bot check by being signed off -- see the failing DCO check for instructions on how to retroactively sign commits