Allow passing a map of issuer -> JWKS to the `jwks` configuration attribute

[anderseknert]

Those who may have multiple valid issuers configured but still want to provide the JWKS data per issuer inside of a bundle (and not fetched via the metadata flow), we should allow passing an object to the jwks object that maps between issuer and JWKS. Currently, the user would have to do this out of band, and merge the keys into a single keys array:

{
  "allowed_issuers": [
    "https://foo.example.com",
    "https://bar.example.com",
  ],
  "jwks": {
    "keys": [
      {
        "kty": "RSA",
        "n": "0uUZ4XpiWu4ds6SxR.....",
        "e": "AQAB"
      },
      { 
        "more keys from all issuers": "here..."
      }
    ]
  }
}

In addition to this, we should allow passing a map of issuers -> keys.

{
  "allowed_issuers": [
    "https://foo.example.com",
    "https://bar.example.com"
  ],
  "jwks": {
    "https://foo.example.com": {
      "keys": [
        {
          "kty": "RSA",
          "n": "0uUZ4XpiWu4ds6SxR.....",
          "e": "AQAB"
        }
      ]
    },
    "https://bar.example.com": {
      "keys": [
        {
          "kty": "RSA",
          "n": "RxS6sd4uWipX4ZUu0.....",
          "e": "AQAB"
        }
      ]
    }
  }
}