Add create/update/delete endpoints for instance SCIM

What does this MR do and why?

• POST/PATCH/DELETE added
• GET endpoints were added separately: !107770 (merged)
• These endpoints mirror the endpoints for Group SCIM: https://gitlab.com/gitlab-org/gitlab/-/blob/7c33e7c4f71de373cd444ec4e684b9592c4ad442/ee/lib/api/scim/group_scim.rb
• There were some differences made for these endpoints, mostly around error handling, because the Group SCIM endpoints need improvement. I've opened an issue for the updates needed for Group SCIM: #387750 (closed)
• Will do a follow-up MR to add docs for these endpoints: #387591 (closed)
• #378599 (closed)

Screenshots or screen recordings

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How to set up and validate locally

Test POST /Users endpoint:

1. Create a scim access token in a rails console and copy the token value

   token = ScimOauthAccessToken.create!
   token.token
   => $YOUR_SECRET_TOKEN

2. Make a curl request using the token

    curl --location --request POST "http://localhost:3000/api/scim/v2/application/Users" --header "Authorization: Bearer $YOUR_SECRET_TOKEN" --header "Content-Type: application/scim+json" --data '{"externalId": "scim_user_2", "userName": "TestUser2Username", "emails": [{"primary": true, "type": "work", "value": "testuser2@example.com"}], "name": { "formatted": "TestFormatedName", "familyName": "LastName2", "givenName": "TestUser2" }, "access_token": "fakeyfakeaccesstoken", "password": "definitelynotarealpassword" }'

3. Response json should look like:

    {"schemas":["urn:ietf:params:scim:schemas:core:2.0:User"],"id":"scim_user_2","active":true,"emails":[{"type":"work","value":"testuser2@example.com","primary":true}],"name":{"formatted":"TestFormatedName","givenName":"TestFormatedName","familyName":""},"meta":{"resourceType":"User"},"userName":"TestUser2Username"}

Test PATCH /Users/:id endpoint:

1. Note the extern_uid of the user created when testing the POST endpoint. We will update a user with extern_uid value of .
2. Make a curl request using the token you created when testing other endpoint. The params are encoded so it is hard to read but this is changing the extern_uid for this user from scim_user_2 to new_extern_uid_2

    curl --location --request PATCH "http://localhost:3000/api/scim/v2/application/Users/scim_user_2?Operations%5B%5D%5Bop%5D=Replace&Operations%5B%5D%5Bpath%5D=id&Operations%5B%5D%5Bvalue%5D=new_extern_uid_2" --header "Content-Type: application/scim+json" --header "Authorization: Bearer $YOUR_SECRET_TOKEN" 

3. Response JSON should be empty in a success case but you can verify the extern_uid update was successful in rails console:

    ScimIdentity.last
   
    => #<ScimIdentity:0x000000012ed6d038
     id: 2,
     group_id: nil,
     user_id: 56,
     created_at: Wed, 11 Jan 2023 00:04:12.171090000 UTC +00:00,
     updated_at: Wed, 11 Jan 2023 00:09:28.521818000 UTC +00:00,
     active: true,
     extern_uid: "new_extern_uid_2">

Test DELETE /Users/:id endpoint:

1. Assuming there is a user with an extern_uid value of new_extern_uid_2

    curl --location --request DELETE "http://localhost:3000/api/scim/v2/application/Users/new_extern_uid_2" --header "Content-Type: application/scim+json" --header "Authorization: Bearer $YOUR_SECRET_TOKEN"

2. Response JSON should be empty in a success case but you can verify that the scim identity was deprovisioned (active: false) in a Rails console:

   ScimIdentity.last
   
   => #<ScimIdentity:0x000000012ed0ca58
    id: 2,
    group_id: nil,
    user_id: 56,
    created_at: Wed, 11 Jan 2023 00:04:12.171090000 UTC +00:00,
    updated_at: Wed, 11 Jan 2023 00:13:51.875750000 UTC +00:00,
    active: false,
    extern_uid: "new_extern_uid_2">

## 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](https://docs.gitlab.com/ee/development/code_review.html#acceptance-checklist) for this MR.

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