Add IsJWT validation function, tests, and README entry (Fixes #502)

[shushantrishav]

Q	A
Documentation	yes
Bugfix	no
BC Break	no
New Feature	yes
RFC	yes
QA	yes

Description

This PR adds a new validator function IsJWT() to the govalidator package.

Why this change is necessary: Currently, JWT strings can only be validated using a manual regex inside a valid:"matches(...)" struct tag, which is not very convenient or readable. As per RFC-7519, a JWT is represented as a sequence of three URL-safe base64-encoded strings separated by periods ('.'). This PR implements a native validation function to improve developer ergonomics and code readability.

What this PR does:

• Implements a new IsJWT() function using a compiled regex pattern.
• Adds corresponding unit tests in validator_test.go.
• Updates the README.md function list to include the new IsJWT() function.

Target branch: master