Add OIDC login support for Google and Microsoft providers

Implements OpenID Connect authentication flow with PKCE, JWT verification, and enterprise-only account restrictions. Adds OIDC service with JWKS caching and state management, HTTP handlers for login/callback flows, GraphQL query for available providers, and sign-in UI integration.

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

pkg/iam/oidc/errors.go

@@ -0,0 +1,99 @@
+// Copyright (c) 2025 Probo Inc <hello@getprobo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package oidc
+
+import (
+	"fmt"
+
+	"go.probo.inc/probo/pkg/coredata"
+)
+
+type ErrProviderNotEnabled struct {
+	Provider coredata.OIDCProvider
+}
+
+func NewProviderNotEnabledError(provider coredata.OIDCProvider) error {
+	return &ErrProviderNotEnabled{Provider: provider}
+}
+
+func (e ErrProviderNotEnabled) Error() string {
+	return fmt.Sprintf("cannot authenticate: OIDC provider %q is not enabled", e.Provider)
+}
+
+type ErrInvalidState struct{}
+
+func NewInvalidStateError() error {
+	return &ErrInvalidState{}
+}
+
+func (e ErrInvalidState) Error() string {
+	return "cannot validate OIDC state: invalid or expired"
+}
+
+type ErrCodeExchange struct {
+	Err error
+}
+
+func NewCodeExchangeError(err error) error {
+	return &ErrCodeExchange{Err: err}
+}
+
+func (e ErrCodeExchange) Error() string {
+	return fmt.Sprintf("cannot exchange authorization code: %v", e.Err)
+}
+
+func (e ErrCodeExchange) Unwrap() error {
+	return e.Err
+}
+
+type ErrIDTokenMissing struct{}
+
+func NewIDTokenMissingError() error {
+	return &ErrIDTokenMissing{}
+}
+
+func (e ErrIDTokenMissing) Error() string {
+	return "cannot extract id_token: not present in token response"
+}
+
+type ErrMissingEmailClaim struct{}
+
+func NewMissingEmailClaimError() error {
+	return &ErrMissingEmailClaim{}
+}
+
+func (e ErrMissingEmailClaim) Error() string {
+	return "cannot extract email: claim missing from id token"
+}
+
+type ErrEmailNotVerified struct{}
+
+func NewEmailNotVerifiedError() error {
+	return &ErrEmailNotVerified{}
+}
+
+func (e ErrEmailNotVerified) Error() string {
+	return "cannot authenticate: email address is not verified by the OIDC provider"
+}
+
+type ErrPersonalAccountNotAllowed struct{}
+
+func NewPersonalAccountNotAllowedError() error {
+	return &ErrPersonalAccountNotAllowed{}
+}
+
+func (e ErrPersonalAccountNotAllowed) Error() string {
+	return "cannot authenticate: personal accounts are not allowed, use an enterprise account"
+}