internal/fwserver: Report semantic equality modification instead of comparing whole values

[samssh]

Related Issue

Fixes #1322 (partially — this PR covers site 3)

Description

ReadResource, CreateResource, UpdateResource and ReadDataSource decided
whether semantic equality had changed anything by deep-comparing the entire
returned value against the value they passed in:

if !semanticEqualityResp.NewData.TerraformValue.Equal(resp.NewState.Raw) {

For a resource with a large set attribute that comparison is O(n²) in the number
of set elements, and it runs on every one of those RPCs. In a CPU profile of a
real provider, a single call to this line accounted for 115s of a 181s profile.

SchemaSemanticEquality already knows whether it replaced a value — it is the
code doing the replacing — so it now reports that in the response and the four
call sites check the flag instead.

Four expectations in schema_semantic_equality_test.go gain Modified: true.
Those literals predate the field; in each of them the proposed value is "new"
and the expected new value is "prior", so the data genuinely changed and the
comparison being replaced reached the same conclusion. Nothing was weakened to
make a test pass.

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

No.

[hashicorp-cla-app]

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