Rust: Add support for MaD sources and sinks with access paths

[hvitved]

Overview

This PR adds support for defining flow sources/sinks for Rust with non-trivial access paths. For example, one may define

extensions:
  - addsTo:
      pack: codeql/rust-all
      extensible: sourceModel
    data:
      - ["repo::test", "crate::my_option_source", "ReturnValue.Variant[crate::option::Option::Some(0)]", "test-source", "manual"]

which models calls to crate::my_option_source as flow sources of kind test-source, but where the origin of tainted data is not directly the result of the call, but rather stored inside Option::Some in the result of the call.

Similarly, one may define a sink restricted to data stored inside Option::Some:

extensions:
  - addsTo:
      pack: codeql/rust-all
      extensible: sinkModel
    data:
      - ["repo::test", "crate::my_option_sink", "Argument[0].Variant[crate::option::Option::Some(0)]", "test-sink", "manual"]

Implementation

The implementation piggy-backs on the existing FlowSummaryImpl library, which already has functionality for specifying flow summaries with non-trivial input/output access paths. For a flow source like my_option_source above, we synthesize a store step from synthetic source nodes to actual calls to my_option_source, and dually for sinks like my_option_sink, we synthesize read steps from arguments to synthetic sink nodes.

The functionality is currently limited to Rust, but other languages should be able to adopt relatively easily.