custom flag type doesn't respect `required` set in a custom flag

[Cristian Dominguez]

Describe the bug
If I have a custom flag with required: true and use it in a command, its type is <T | undefined>.

To Reproduce

1. clone this branch of sf-plugin-api: https://github.com/cristiand391/sf-plugin-api/tree/cd%2Foclif-required-flag
2. see target-org type is (property) 'target-org': OptionFlag<Org, CustomOptions> org api command
3. Delete this line: https://github.com/cristiand391/sf-plugin-api/blob/deb78ab0eccd81ba9043760d7d4fe9d36bb2b474/src/commands/org/api.ts#L39
4. target-org type now is: (property) 'target-org': OptionFlag<Org | undefined, CustomOptions>

requiredOrgFlag already sets required: true so we shouldn't need to set required again:
https://github.com/salesforcecli/sf-plugins-core/blob/c43456953f5fb14bb64d1cc72f84c78c2c211088/src/flags/orgFlags.ts#L154

For comparision, I added a two flags in the org api command:
https://github.com/cristiand391/sf-plugin-api/blob/deb78ab0eccd81ba9043760d7d4fe9d36bb2b474/src/commands/org/api.ts#L16-L23

lalaFlag is a string flag, testFlag is a custom flag and both set required: true, if you check their types here:
https://github.com/cristiand391/sf-plugin-api/blob/deb78ab0eccd81ba9043760d7d4fe9d36bb2b474/src/commands/org/api.ts#L31-L32

you can see lalaFlag respects required:true:
(property) lala: OptionFlag<string, CustomOptions>
and testFlag doesn't:
(property) test: OptionFlag<string | undefined, CustomOptions>

Expected behavior
custom flag type should recognize when required is set.

Environment (please complete the following information):

• OS & version: macos ventura
• Shell/terminal & version zsh/alacritty

Additional context
We've been manually setting required in our plugins:
https://github.com/search?q=org%3Asalesforcecli+%2FFlags%5C.requiredOrg%2F+%2Frequired%3A+true%2F&type=code

You can see that required is actually passed (see below the help text org --target-org after required: true is deleted in org api), the issue seems to be only the flag type: