Support max_byte_size settings for instance SAML

What does this MR do and why?

We had saml_message_max_byte_size settings which was only applicable for group SAML. This MR enables the instance same config to configure maximum message size

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Verified via omnibus:

• Triggered a docker build in https://gitlab.com/gitlab-org/build/omnibus-gitlab-mirror/-/jobs/5068599028 and copied the built and deployed docker image registry.gitlab.com/gitlab-org/build/omnibus-gitlab-mirror/gitlab-ee:56b13284cf9f42aebd712b62055bc84b310f33a7

• Shell time:

docker run --name gitlab-omnibus-mr-130515 -p 2323:2323 -d --env GITLAB_OMNIBUS_CONFIG="external_url 'http://10.23.0.5:2323/'; gitlab_rails['initial_root_password'] = 'fivelife';" registry.gitlab.com/gitlab-org/build/omnibus-gitlab-mirror/gitlab-ee:56b13284cf9f42aebd712b62055bc84b310f33a7`

# wait for it...

# Connect to running GitLab docker instance
docker exec -it gitlab-omnibus-mr-130515 bash

root@070d4d17111f:/# gitlab-rails console

# A lot of warnings about pattern matching feature in remote_development :disappointed: 

--------------------------------------------------------------------------------
 Ruby:         ruby 3.0.6p216 (2023-03-30 revision 23a532679b) [x86_64-linux]
 GitLab:       16.4.0-pre (56b13284cf9) EE
 GitLab Shell: 14.27.0
 PostgreSQL:   13.11
------------------------------------------------------------[ booted in 33.49s ]
Loading production environment (Rails 7.0.6)
irb(main):001:0>
irb(main):002:0> OmniAuth::Strategies::SAML.default_options['message_max_bytesize']
=> 250000

🎉

✅

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #376721 (closed)