Fixes the following error, when running `npm run dev` in the backend:

```
/home/tal/Code/NextStep/nextstep-backend/node_modules/ts-node/src/index.ts:859
    return new TSError(diagnosticText, diagnosticCodes, diagnostics);
           ^
TSError: ⨯ Unable to compile TypeScript:
src/models/user_model.ts:34:13 - error TS2322: Type 'unknown' is not assignable to type 'string'.

34             id: ret._id,
               ~~

  src/types/user_types.ts:13:5
    13     id: string;
           ~~
    The expected type comes from property 'id' which is declared here on type 'UserData'
src/models/user_model.ts:35:13 - error TS2322: Type 'unknown' is not assignable to type 'string'.

35             username: ret.username,
               ~~~~~~~~

  src/types/user_types.ts:14:5
    14     username: string;
           ~~~~~~~~
    The expected type comes from property 'username' which is declared here on type 'UserData'
src/models/user_model.ts:36:13 - error TS2322: Type 'unknown' is not assignable to type 'string'.

36             email: ret.email,
               ~~~~~

  src/types/user_types.ts:15:5
    15     email: string;
           ~~~~~
    The expected type comes from property 'email' which is declared here on type 'UserData'
src/models/user_model.ts:37:13 - error TS2322: Type 'unknown' is not assignable to type 'string'.

37             password: ret.password,
               ~~~~~~~~

  src/types/user_types.ts:16:5
    16     password: string;
           ~~~~~~~~
    The expected type comes from property 'password' which is declared here on type 'UserData'
src/models/user_model.ts:38:13 - error TS2322: Type 'unknown' is not assignable to type 'string | undefined'.

38             imageFilename: ret?.imageFilename,
               ~~~~~~~~~~~~~

  src/types/user_types.ts:17:5
    17     imageFilename?: string;
           ~~~~~~~~~~~~~
    The expected type comes from property 'imageFilename' which is declared here on type 'UserData'
src/models/user_model.ts:39:13 - error TS2322: Type 'unknown' is not assignable to type 'string | undefined'.

39             createdAt: ret.createdAt,
               ~~~~~~~~~

  src/types/user_types.ts:18:5
    18     createdAt?: string,
           ~~~~~~~~~
    The expected type comes from property 'createdAt' which is declared here on type 'UserData'
src/models/user_model.ts:40:13 - error TS2322: Type 'unknown' is not assignable to type 'string | undefined'.

40             updatedAt: ret.updatedAt
               ~~~~~~~~~

  src/types/user_types.ts:19:5
    19     updatedAt?: string,
           ~~~~~~~~~
    The expected type comes from property 'updatedAt' which is declared here on type 'UserData'

    at createTSError (/home/tal/Code/NextStep/nextstep-backend/node_modules/ts-node/src/index.ts:859:12)
    at reportTSError (/home/tal/Code/NextStep/nextstep-backend/node_modules/ts-node/src/index.ts:863:19)
    at getOutput (/home/tal/Code/NextStep/nextstep-backend/node_modules/ts-node/src/index.ts:1077:36)
    at Object.compile (/home/tal/Code/NextStep/nextstep-backend/node_modules/ts-node/src/index.ts:1433:41)
    at Module.m._compile (/home/tal/Code/NextStep/nextstep-backend/node_modules/ts-node/src/index.ts:1617:30)
    at node:internal/modules/cjs/loader:1895:10
    at Object.require.extensions.<computed> [as .ts] (/home/tal/Code/NextStep/nextstep-backend/node_modules/ts-node/src/index.ts:1621:12)
    at Module.load (node:internal/modules/cjs/loader:1465:32)
    at Function._load (node:internal/modules/cjs/loader:1282:12)
    at TracingChannel.traceSync (node:diagnostics_channel:322:14) {
  diagnosticCodes: [
    2322, 2322,
    2322, 2322,
    2322, 2322,
    2322
  ]
}
[nodemon] app crashed - waiting for file changes before starting...
```

**Problem**
In `user_model.ts`, the `transform` function is typed
to return `UserData`, which expects all fields to be
of type `string` (or `string | undefined` for some).
However, the `ret` object passed to the transform
function is not strongly typed—it is inferred as
`any` or `unknown` by TypeScript, especially when
using strict settings.

This fixes all the models` `transform` function.

Signed-off-by: Tal Jacob <taljacob2@gmail.com>