Add aws auth backend config identity

[fairclothjm]

This change introduces the aws_auth_backend_config_identity resource so that users can configure the way the AWS auth backend interacts with the Identity store. This is adapted from the community PR #1248

Release note for CHANGELOG:

`resource/aws_auth_backend_config_identity`: Configure how Vault interacts with the Identity store for AWS authentication

Output from acceptance testing:

$ TESTARGS="--run TestAccAwsAuthBackendConfigIdentity" make testacc
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test --run TestAccAwsAuthBackendConfigIdentity -timeout 30m ./...
?   	github.com/hashicorp/terraform-provider-vault	[no test files]
?   	github.com/hashicorp/terraform-provider-vault/cmd/coverage	[no test files]
?   	github.com/hashicorp/terraform-provider-vault/cmd/generate	[no test files]
ok  	github.com/hashicorp/terraform-provider-vault/codegen	(cached) [no tests to run]
?   	github.com/hashicorp/terraform-provider-vault/generated	[no test files]
ok  	github.com/hashicorp/terraform-provider-vault/generated/datasources/transform/decode	1.501s [no tests to run]
ok  	github.com/hashicorp/terraform-provider-vault/generated/datasources/transform/encode	2.873s [no tests to run]
ok  	github.com/hashicorp/terraform-provider-vault/generated/resources/transform/alphabet	1.318s [no tests to run]
ok  	github.com/hashicorp/terraform-provider-vault/generated/resources/transform/role	2.225s [no tests to run]
ok  	github.com/hashicorp/terraform-provider-vault/generated/resources/transform/template	3.068s [no tests to run]
ok  	github.com/hashicorp/terraform-provider-vault/generated/resources/transform/transformation	1.360s [no tests to run]
?   	github.com/hashicorp/terraform-provider-vault/helper	[no test files]
?   	github.com/hashicorp/terraform-provider-vault/internal/consts	[no test files]
ok  	github.com/hashicorp/terraform-provider-vault/internal/identity/entity	(cached) [no tests to run]
?   	github.com/hashicorp/terraform-provider-vault/internal/identity/group	[no test files]
?   	github.com/hashicorp/terraform-provider-vault/internal/identity/mfa	[no test files]
?   	github.com/hashicorp/terraform-provider-vault/internal/pki	[no test files]
ok  	github.com/hashicorp/terraform-provider-vault/internal/provider	(cached) [no tests to run]
?   	github.com/hashicorp/terraform-provider-vault/schema	[no test files]
ok  	github.com/hashicorp/terraform-provider-vault/testutil	(cached) [no tests to run]
ok  	github.com/hashicorp/terraform-provider-vault/util	(cached) [no tests to run]
ok  	github.com/hashicorp/terraform-provider-vault/vault	4.715s

[vinay-gopalan]

Looks great! Had a couple comments about using constants and adding the namespace field to the docs, but otherwise looks good to go; thanks for adding this! 😄