fix: improve password validation flow (#15132)

Refers to #14984 

Currently, password validation is done backend side and is not explicit
enough so it can be painful to create first users.
We'd like to make this validation easier - but also duplicate it
frontend side to make it smoother.

Flows involved : 
- First user set password
- New user set password
- Change password

---------

Co-authored-by: BrunoQuaresma <bruno_nonato_quaresma@hotmail.com>

coderd/apidoc/swagger.json

@@ -4737,6 +4737,39 @@
 				}
 			}
 		},
+		"/users/validate-password": {
+			"post": {
+				"security": [
+					{
+						"CoderSessionToken": []
+					}
+				],
+				"consumes": ["application/json"],
+				"produces": ["application/json"],
+				"tags": ["Authorization"],
+				"summary": "Validate user password",
+				"operationId": "validate-user-password",
+				"parameters": [
+					{
+						"description": "Validate user password request",
+						"name": "request",
+						"in": "body",
+						"required": true,
+						"schema": {
+							"$ref": "#/definitions/codersdk.ValidateUserPasswordRequest"
+						}
+					}
+				],
+				"responses": {
+					"200": {
+						"description": "OK",
+						"schema": {
+							"$ref": "#/definitions/codersdk.ValidateUserPasswordResponse"
+						}
+					}
+				}
+			}
+		},
 		"/users/{user}": {
 			"get": {
 				"security": [
@@ -12817,6 +12850,26 @@
 				"UserStatusSuspended"
 			]
 		},
+		"codersdk.ValidateUserPasswordRequest": {
+			"type": "object",
+			"required": ["password"],
+			"properties": {
+				"password": {
+					"type": "string"
+				}
+			}
+		},
+		"codersdk.ValidateUserPasswordResponse": {
+			"type": "object",
+			"properties": {
+				"details": {
+					"type": "string"
+				},
+				"valid": {
+					"type": "boolean"
+				}
+			}
+		},
 		"codersdk.ValidationError": {
 			"type": "object",
 			"required": ["detail", "field"],