fix: add gpsi and snssais query parameter check for nf_discovery

[Zach1113]

This PR fixes two runtime panic issues in NF Discovery:

Issue #757:

• Resolved a runtime panic that occurs when the gpsi parameter is shorter than 7 characters.
• This fix uses the new functionality in free5gc/util/validator.
• Note: This validation relies on a new feature in theutil package; it will become fully functional once the corresponding util PR is merged.

Issue #758:

• Resolved a runtime panic triggered by an empty snssais query parameter.
• Added a length and format check to validate the snssais string before performing slice operations.