Support multiple issuers in JwtBearerConfigureOptions

[martincostello]

Is there an existing issue for this?

• I have searched the existing issues

Is your feature request related to a problem? Please describe the problem.

When configuring default bearer authentication options from configuration, only one valid issuer is supported.

aspnetcore/src/Security/Authentication/JwtBearer/src/JwtBearerConfigureOptions.cs

Lines 38 to 55 in d7d7deb

	var configSection = _authenticationConfigurationProvider.GetAuthenticationSchemeConfiguration(name);
	if (configSection is null || !configSection.GetChildren().Any())
	{
	return;
	}
	var issuer = configSection["ClaimsIssuer"];
	var audiences = configSection.GetSection("Audiences").GetChildren().Select(aud => aud.Value).ToArray();
	options.TokenValidationParameters = new()
	{
	ValidateIssuer = issuer is not null,
	ValidIssuers = new[] { issuer },
	ValidateAudience = audiences.Length > 0,
	ValidAudiences = audiences,
	ValidateIssuerSigningKey = true,
	IssuerSigningKey = GetIssuerSigningKey(_configuration, issuer),
	};

Describe the solution you'd like

The configuration schema should support the configuration of multiple valid issuers, like it does for audiences.

aspnetcore/src/Security/Authentication/JwtBearer/src/JwtBearerConfigureOptions.cs

Line 46 in d7d7deb

var audiences = configSection.GetSection("Audiences").GetChildren().Select(aud => aud.Value).ToArray();

Additional context

See #41956 (comment).

/cc @DamianEdwards

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