Support sender verification

[kenzo0107]

Provide a resource to manage sender verification.

[kenzo0107]

$ make testacc
TF_ACC=1 go test ./... -v  -timeout 120m
?       github.com/kenzo0107/terraform-provider-sendgrid        [no test files]
?       github.com/kenzo0107/terraform-provider-sendgrid/examples/resources/sendgrid_link_branding/tmp       [no test files]
?       github.com/kenzo0107/terraform-provider-sendgrid/flex   [no test files]
=== RUN   TestAccAPIKeyDataSource
--- PASS: TestAccAPIKeyDataSource (9.53s)
=== RUN   TestAccAPIKeyResource
--- PASS: TestAccAPIKeyResource (17.11s)
=== RUN   TestAccLinkBrandingDataSource
--- PASS: TestAccLinkBrandingDataSource (14.75s)
=== RUN   TestAccLinkBrandingResource
--- PASS: TestAccLinkBrandingResource (23.04s)
=== RUN   TestAccSenderAuthenticationDataSource
--- PASS: TestAccSenderAuthenticationDataSource (14.42s)
=== RUN   TestAccSenderAuthenticationResource
--- PASS: TestAccSenderAuthenticationResource (20.92s)
=== RUN   TestAccSenderVerificationDataSource
--- PASS: TestAccSenderVerificationDataSource (14.83s)
=== RUN   TestAccSenderVerificationResource
--- PASS: TestAccSenderVerificationResource (23.25s)
=== RUN   TestAccSubuserDataSource
--- PASS: TestAccSubuserDataSource (9.75s)
=== RUN   TestAccSubuserResource
--- PASS: TestAccSubuserResource (16.37s)
=== RUN   TestAccTeammateDataSource
--- PASS: TestAccTeammateDataSource (9.92s)
=== RUN   TestAccTeammateResource
--- PASS: TestAccTeammateResource (17.24s)
PASS
ok      github.com/kenzo0107/terraform-provider-sendgrid/internal/provider      191.619s