provider/aws: Add validation for ECR repository name

[radeksimko]

I have also moved all existing validation functions into a separate file.

Next step (assuming different PR) can be turning the inline functions that left into standalone functions + adding some tests.

Test plan

$ make test TEST=./builtin/providers/aws TESTARGS='-run=TestValidate'

$ make testacc TEST=./builtin/providers/aws TESTARGS='-run=AWSEcr'

==> Checking that code complies with gofmt requirements...
go generate ./...
TF_ACC=1 go test ./builtin/providers/aws -v -run=AWSEcr -timeout 90m
=== RUN   TestAccAWSEcrRepositoryPolicy_basic
--- PASS: TestAccAWSEcrRepositoryPolicy_basic (8.84s)
=== RUN   TestAccAWSEcrRepository_basic
--- PASS: TestAccAWSEcrRepository_basic (7.67s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    16.534s

[phinze]

LGTM!

[ghost]

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