Fix race condition on duplicate entity alias creation

[benashz]

Fix race condition on duplicate entity alias creation

The creation of entity aliases should be serialized, since we want to
handle the case when the user is attempting to create duplicate entity
aliases during the same Teraform execution. A duplicate entity alias
shares the same mount accessor and name with another entity alias.

The fix is to lock the /identity/entity-alias/id path on all create,
update, and delete operations. This allows the provider to reliably
detect the duplicate entity alias condition, and prevent duplicate
alias creation.

Other fixes:

• all CRUD functions are of the *Context type, and can return meaningful
  diag.Diagnostics
• created a new entity package under internal/identity/entity with some
  helper functions

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Closes #669

Release note for CHANGELOG:

Output from acceptance testing:

$ make testacc TESTARGS='-v -test.run TestAccIdentityEntity*'

==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test -v -v -test.run TestAccIdentityEntity* -timeout 30m ./...

ok      github.com/hashicorp/terraform-provider-vault/util      (cached) [no tests to run]
=== RUN   TestAccIdentityEntityAlias
--- PASS: TestAccIdentityEntityAlias (3.08s)
=== RUN   TestAccIdentityEntityAliasDuplicateFlow
--- PASS: TestAccIdentityEntityAliasDuplicateFlow (6.13s)
=== RUN   TestAccIdentityEntityAlias_Update
--- PASS: TestAccIdentityEntityAlias_Update (3.06s)
=== RUN   TestAccIdentityEntityAlias_Metadata
--- PASS: TestAccIdentityEntityAlias_Metadata (3.07s)
=== RUN   TestAccIdentityEntityPoliciesExclusive
--- PASS: TestAccIdentityEntityPoliciesExclusive (2.98s)
=== RUN   TestAccIdentityEntityPoliciesNonExclusive
--- PASS: TestAccIdentityEntityPoliciesNonExclusive (2.96s)
=== RUN   TestAccIdentityEntity
--- PASS: TestAccIdentityEntity (1.58s)
=== RUN   TestAccIdentityEntityUpdate
--- PASS: TestAccIdentityEntityUpdate (2.81s)
=== RUN   TestAccIdentityEntityUpdateRemoveValues
--- PASS: TestAccIdentityEntityUpdateRemoveValues (2.82s)
=== RUN   TestAccIdentityEntityUpdateRemovePolicies
--- PASS: TestAccIdentityEntityUpdateRemovePolicies (2.85s)
PASS
ok      github.com/hashicorp/terraform-provider-vault/vault     31.782s


...

[vinay-gopalan]

LGTM! Great work 😄