how do I convert a string into a number? and use number checks

#### Context

```
joi@17.6.0
v16.14.2
Darwin 5c52309d33e3 20.6.0 Darwin Kernel Version 20.6.0: Tue Apr 19 21:04:45 PDT 2022; root:xnu-7195.141.29~1/RELEASE_X86_64 x86_64 i386 Darwin
```

#### How can we help?

I'm trying to check whether one converted object is greater than another, problem is it's not converting

```
1) env
custom conversion:
ValidationError: "RETRY_TIMEOUT_DURATION" must be a number
at Object.exports.process (/Users/nqy642/IdeaProjects/exceptionsone/node_modules/joi/lib/errors.js:193:16)
at Object.internals.entry (/Users/nqy642/IdeaProjects/exceptionsone/node_modules/joi/lib/validator.js:153:26)
at Object.exports.entry (/Users/nqy642/IdeaProjects/exceptionsone/node_modules/joi/lib/validator.js:27:30)
at internals.Base.validate (/Users/nqy642/IdeaProjects/exceptionsone/node_modules/joi/lib/base.js:548:26)
at Object.internals.assert (/Users/nqy642/IdeaProjects/exceptionsone/node_modules/joi/lib/index.js:225:27)
at Object.attempt (/Users/nqy642/IdeaProjects/exceptionsone/node_modules/joi/lib/index.js:107:26)
at Object.validateRequired (/Users/nqy642/IdeaProjects/exceptionsone/packages/util/src/validate.ts:20:37)
at Context. (src/framework/util/env.spec.ts:32:19)
at processImmediate (node:internal/timers:466:21)
at process.topLevelDomainCallback (node:domain:152:15)
at process.callbackTrampoline (node:internal/async_hooks:128:24)
```

```ts
import Joi from 'joi'
import parse from 'parse-duration'
import { Primitive, SafeDictionary } from 'ts-essentials'

export type EnvKey =
| 'RETRY_TIMEOUT_DURATION'
| 'AXIOS_TIMEOUT_DURATION'

export type Env = SafeDictionary, EnvKey> & {
RETRY_TIMEOUT_DURATION: number
AXIOS_TIMEOUT_DURATION: number
}

const convertDuration = (envVar: string): number => parse(envVar)

export const envValidator = Joi.object({
AXIOS_TIMEOUT_DURATION: Joi.number().empty().default(30000).custom(convertDuration),
RETRY_TIMEOUT_DURATION: Joi.number()
.greater(Joi.ref('AXIOS_TIMEOUT_DURATION'))
.empty()
.default(40000)
.custom(convertDuration),
})
```
```ts
it('custom conversion', () => {
const environment = {
RETRY_TIMEOUT_DURATION: '40s',
}
const valid = Joi.attempt(environment, envValidator)

expect(valid.RETRY_TIMEOUT_DURATION).to.be.eq(40000)
})
```

if I use `Joi.string()` it seems to work except that I can't use `.greater()` because it's not on string.