JWT for customers-dot graphql proxy

What does this MR do?

For: customers-gitlab-com#3000 (closed)
customers-dot integration: https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/3511

This MR introduces a new JWT that will be used to authenticate/authorize a GitLab user's CustomersDot resources. With this, we'll be able to query CustomerDot for the current GitLab users resources seamlessly via the CustomersDot graphql proxy.

An example flow:

1. On gitlab, frontend makes request to /-/customers_dot/proxy/graphql
2. At the controller, we create a JWT containing the currently logged in user, attach it to the request headers and forward the request to customersdot
3. CustomersDot verifies the JWT, queries in the context of the JWT user and returns results
4. Original frontend request gets response

migrations

up

== 20210630222522 AddCustomersDotJwtSigningKeyToApplicationSettings: migrating 
-- add_column(:application_settings, :encrypted_customers_dot_jwt_signing_key, :text)
   -> 0.0047s
-- add_column(:application_settings, :encrypted_customers_dot_jwt_signing_key_iv, :text)
   -> 0.0014s
== 20210630222522 AddCustomersDotJwtSigningKeyToApplicationSettings: migrated (0.0062s) 

== 20210630224625 GenerateCustomersDotJwtSigningKey: migrating ================
== 20210630224625 GenerateCustomersDotJwtSigningKey: migrated (0.5486s) =======

down

== 20210630224625 GenerateCustomersDotJwtSigningKey: reverting ================
== 20210630224625 GenerateCustomersDotJwtSigningKey: reverted (0.3851s) =======

== 20210630222522 AddCustomersDotJwtSigningKeyToApplicationSettings: reverting 
-- remove_column(:application_settings, :encrypted_customers_dot_jwt_signing_key_iv, :text)
   -> 0.0045s
-- remove_column(:application_settings, :encrypted_customers_dot_jwt_signing_key, :text)
   -> 0.0015s
== 20210630222522 AddCustomersDotJwtSigningKeyToApplicationSettings: reverted (0.0108s) 

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team