Fix secret_binary in secret_version datasource

[wedge-jarrad]

Base64 encode the returned binary secret. This is consistant with how the
binary secret is supplied to the resource and with how the resource itself
represents the binary data. Simply casting the binary data to a string doesn't
appear to result in anything usable.

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Closes #25603

Output from acceptance testing:

$ gmake testacc TESTS='TestAccSecretsManagerSecretVersionDataSource*' PKG=secretsmanager
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./internal/service/secretsmanager/... -v -count 1 -parallel 20 -run='TestAccSecretsManagerSecretVersionDataSource*'  -timeout 180m
=== RUN   TestAccSecretsManagerSecretVersionDataSource_basic
=== PAUSE TestAccSecretsManagerSecretVersionDataSource_basic
=== RUN   TestAccSecretsManagerSecretVersionDataSource_binary
=== PAUSE TestAccSecretsManagerSecretVersionDataSource_binary
=== RUN   TestAccSecretsManagerSecretVersionDataSource_versionID
=== PAUSE TestAccSecretsManagerSecretVersionDataSource_versionID
=== RUN   TestAccSecretsManagerSecretVersionDataSource_versionStage
=== PAUSE TestAccSecretsManagerSecretVersionDataSource_versionStage
=== CONT  TestAccSecretsManagerSecretVersionDataSource_basic
=== CONT  TestAccSecretsManagerSecretVersionDataSource_versionID
=== CONT  TestAccSecretsManagerSecretVersionDataSource_binary
=== CONT  TestAccSecretsManagerSecretVersionDataSource_versionStage
--- PASS: TestAccSecretsManagerSecretVersionDataSource_versionStage (14.90s)
--- PASS: TestAccSecretsManagerSecretVersionDataSource_binary (14.91s)
--- PASS: TestAccSecretsManagerSecretVersionDataSource_versionID (14.93s)
--- PASS: TestAccSecretsManagerSecretVersionDataSource_basic (15.16s)
PASS
ok  	github.com/hashicorp/terraform-provider-aws/internal/service/secretsmanager	15.246s