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>

docs/reference/api/schemas.md

@@ -6446,6 +6446,36 @@ If the schedule is empty, the user will be updated to use the default schedule.|
 | `dormant`   |
 | `suspended` |
 
+## codersdk.ValidateUserPasswordRequest
+
+```json
+{
+	"password": "string"
+}
+```
+
+### Properties
+
+| Name       | Type   | Required | Restrictions | Description |
+| ---------- | ------ | -------- | ------------ | ----------- |
+| `password` | string | true     |              |             |
+
+## codersdk.ValidateUserPasswordResponse
+
+```json
+{
+	"details": "string",
+	"valid": true
+}
+```
+
+### Properties
+
+| Name      | Type    | Required | Restrictions | Description |
+| --------- | ------- | -------- | ------------ | ----------- |
+| `details` | string  | false    |              |             |
+| `valid`   | boolean | false    |              |             |
+
 ## codersdk.ValidationError
 
 ```json