internal: modify structaccess to accept PathNode instead of dyn.Path

[denik]

Changes

• Modify structaccess.Get/Validate to accept PathNode rather than dyn.Path. It is also flexible with regard to dot or bracket syntax, both are accepted, so "resources.jobs.foo" would still work even though jobs is a map.
• Add PathNode methods: Len(), Prefix(N), SkipPrefix(N)

Depends on #3631

Why

Needed for serialized bundle plan. PathNode is going to come from the plan and then passed directly to structaccess.

Tests

Unit tests.

[eng-dev-ecosystem-bot]

Run: 17860808232

	Env	✅​pass	🔄​flaky	🙈​skip
✅​	aws linux	311		528
✅​	aws windows	312		527
✅​	aws-ucws linux	423		426
✅​	aws-ucws windows	424		425
✅​	azure linux	311		527
✅​	azure windows	312		526
🔄​	azure-ucws linux	416	7	425
🔄​	azure-ucws windows	420	4	424
✅​	gcp linux	310		529
✅​	gcp windows	311		528

8 failing tests:

Test Name	azure-ucws linux	azure-ucws windows
TestAccept/bundle/deploy/dashboard/detect-change	🔄​flaky	✅​pass
TestAccept/bundle/deploy/dashboard/generate_inplace	🔄​flaky	✅​pass
TestAccept/bundle/deploy/dashboard/nested-folders	🔄​flaky	✅​pass
TestAccept/bundle/deploy/dashboard/simple	🔄​flaky	✅​pass
TestAccept/bundle/deploy/dashboard/simple_outside_bundle_root	🔄​flaky	🔄​flaky
TestAccept/bundle/deploy/dashboard/simple_syncroot	🔄​flaky	🔄​flaky
TestAccept/bundle/deployment/bind/dashboard	✅​pass	🔄​flaky
TestDashboardAssumptions_WorkspaceImport	🔄​flaky	🔄​flaky

[denik]

Superseded by #3640