feat: add @IsStrictlyOptional() decorator filtering only missing undefined values

[ehenon]

Description

As mentioned/discussed in some issues and PRs, proposal to add a new decorator based on the same philosophy as @IsOptional(), but filtering only values that are actually missing (=== undefined). Equivalent to @ValidateIf((o, v) => v !== undefined), but more direct and explicit.

Checklist

• the pull request title describes what this PR does (not a vague title like Update index.md)
• the pull request targets the default branch of the repository (develop)
• the code follows the established code style of the repository
  • npm run prettier:check passes
  • npm run lint:check passes
• tests are added for the changes I made (if any source code was modified)
• documentation added or updated
• I have run the project locally and verified that there are no errors

References

references #491, #2615, #686