feat(provider): Implement boundary_credential_username_password_domain resource

This resource creates a static "Username, Password & Domain" credential in
Boundary.

The "username" field is required. It can also contain the domain in one of two
formats:
  - "username@domain.com"
  - "domain.com\username"

The "password" field is required.

The "domain" field is optional but the domain data itself is required (ie. it
must be provided as part of the username or in the dedicated "domain" field)

Co-authored-by: April-May <18632637+AprilMay0@users.noreply.github.com>

examples/resources/boundary_credential_username_password_domain/import.sh

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+terraform import boundary_credential_username_password_domain.example_username_password_domain <my-id>

examples/resources/boundary_credential_username_password_domain/resource.tf

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+resource "boundary_scope" "org" {
+  name                     = "organization_one"
+  description              = "global scope"
+  scope_id                 = "global"
+  auto_create_admin_role   = true
+  auto_create_default_role = true
+}
+
+resource "boundary_scope" "project" {
+  name                   = "project_one"
+  description            = "My first scope!"
+  scope_id               = boundary_scope.org.id
+  auto_create_admin_role = true
+}
+
+resource "boundary_credential_store_static" "example" {
+  name        = "example_static_credential_store"
+  description = "My first static credential store!"
+  scope_id    = boundary_scope.project.id
+}
+
+resource "boundary_credential_username_password_domain" "example" {
+  name                = "example_username_password_domain"
+  description         = "My first username password domain credential!"
+  credential_store_id = boundary_credential_store_static.example.id
+  username            = "my-username"
+  password            = "my-password"
+  domain              = "my-domain"
+}