Update OpenIddictParameter to offer ImmutableArray<string> conversions instead of string[] and clone JsonNode objects to guarantee immutability

[kevinchalet]

This PR introduces the following changes:

• It replaces all the OpenIddictMessage/OpenIddictParameter constructors and static operators using string?[]? objects by equivalents using ImmutableArray<string?> or ImmutableArray<string?>?.
• The OpenIddictRequest.Audiences and OpenIddictRequest.Resources properties have been updated to use ImmutableArray<string?>? instead of string?[]?, which will fully users from trying to do unsupported things like context.Request.Audiences[2] = "overridden audience" (which may or may not currently work depending on the actual CLR type of the parameter value).
• For similar reasons, JsonNode instances are now cloned by OpenIddictParameter's constructor and cloned by the JsonNode? conversion operator.
• It removes the OpenIddictParameter.Value property and replaces it by a new OpenIddictParameter.GetRawValue() to discourage using this very-low-level API.
• The GetDestinations()/SetDestinations() now use ImmutableDictionary<string, ImmutableArray<string>> instead of ImmutableDictionary<string, string[]> for consistency with the previous changes.
• The OpenIddictParameter structure was updated to use recently-introduced APIs like JsonNode.DeepEquals(), JsonElement.DeepEquals() or JsonElement.GetPropertyCount().
• New StringValues conversion operators have been added to the OpenIddictParameter primitive.
• The OpenIddictRequest.Claims and OpenIddictRequest.Registration properties are now of type JsonObject instead of JsonElement, which should make these properties easier to use.

[kevinchalet]

Note: since this PR introduces a bunch of breaking changes, it will need to be properly documented in the 6.x -> 7.x migration guide.