Validate token has key algorithm fixed to RSA-SHA256

### Describe the bug

The grant manager class has the signature algorithm fixed at RSA-SHA256. You can see that [here](https://github.com/keycloak/keycloak-nodejs-connect/blob/main/middleware/auth-utils/grant-manager.js#L443).

Keycloak allows us to change that algorithm. If I do that, the validate function will reject every token, even if it is valid, because it will attempt to use the wrong algorithm.

### Version

keycloak: 15.0.2
keycloak-connect: 15.0.2

### Expected behavior

Keycloak correctly validates tokens signed with different algorithms

### Actual behavior

Keycloak rejects all tokens except those signed with RSA-SHA256

### How to Reproduce?

Change access token signature algorithm to anything other than RS256, generate a token and attempt to validate using this lib.

### Anything else?

A good fix would be to allow the algorithm to be configurable, and have RSA-SHA256 as the default value to avoid a breaking change.

Start in middleware/auth-utils/grant-manager.js around line 443 and trace how the validate function selects the token signature algorithm. Reproduce the issue with a Keycloak token using an algorithm other than RS256, then verify that configurable algorithms work while RSA-SHA256 remains the default.

Written by the indexing model from the issue text.