Support SAML authentication (#25165)

Closes https://github.com/go-gitea/gitea/issues/5512

This PR adds basic SAML support
- Adds SAML 2.0 as an auth source
- Adds SAML configuration documentation
- Adds integration test:
- Use bare-bones SAML IdP to test protocol flow and test account is
linked successfully (only runs on Postgres by default)
- Adds documentation for configuring and running SAML integration test
locally

Future PRs:
- Support group mapping
- Support auto-registration (account linking)

Co-Authored-By: @jackHay22

---------

Co-authored-by: jackHay22 <jack@allspice.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: KN4CK3R <admin@oldschoolhack.me>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: morphelinho <morphelinho@users.noreply.github.com>
Co-authored-by: Zettat123 <zettat123@gmail.com>
Co-authored-by: Yarden Shoham <git@yardenshoham.com>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: silverwind <me@silverwind.io>

go.mod

@@ -91,6 +91,8 @@ require (
 	github.com/quasoft/websspi v1.1.2
 	github.com/redis/go-redis/v9 v9.4.0
 	github.com/robfig/cron/v3 v3.0.1
+	github.com/russellhaering/gosaml2 v0.9.1
+	github.com/russellhaering/goxmldsig v1.3.0
 	github.com/santhosh-tekuri/jsonschema/v5 v5.3.1
 	github.com/sassoftware/go-rpmutils v0.2.1-0.20240124161140-277b154961dd
 	github.com/sergi/go-diff v1.3.1
@@ -143,6 +145,7 @@ require (
 	github.com/anmitsu/go-shlex v0.0.0-20200514113438-38f4b401e2be // indirect
 	github.com/asaskevich/govalidator v0.0.0-20230301143203-a9d515a09cc2 // indirect
 	github.com/aymerick/douceur v0.2.0 // indirect
+	github.com/beevik/etree v1.1.0 // indirect
 	github.com/beorn7/perks v1.0.1 // indirect
 	github.com/bits-and-blooms/bitset v1.13.0 // indirect
 	github.com/blevesearch/bleve_index_api v1.1.5 // indirect
@@ -216,6 +219,7 @@ require (
 	github.com/imdario/mergo v0.3.16 // indirect
 	github.com/jbenet/go-context v0.0.0-20150711004518-d14ea06fba99 // indirect
 	github.com/jessevdk/go-flags v1.5.0 // indirect
+	github.com/jonboulle/clockwork v0.3.0 // indirect
 	github.com/josharian/intern v1.0.0 // indirect
 	github.com/kevinburke/ssh_config v1.2.0 // indirect
 	github.com/klauspost/pgzip v1.2.6 // indirect
@@ -225,6 +229,7 @@ require (
 	github.com/magiconair/properties v1.8.7 // indirect
 	github.com/mailru/easyjson v0.7.7 // indirect
 	github.com/markbates/going v1.0.3 // indirect
+	github.com/mattermost/xml-roundtrip-validator v0.1.0 // indirect
 	github.com/mattn/go-colorable v0.1.13 // indirect
 	github.com/mattn/go-runewidth v0.0.15 // indirect
 	github.com/mholt/acmez v1.2.0 // indirect