Configure Proxy CookieName via Env Var

[alexbakerdev]

Problem

See #197

Solution

Add a COOKIE_NAME env var to the option struct

Notes

It looks like there are already some tests around custom cookie name values, and I couldn't find any tests based around environment variables I couldn't find any, but if there are some tests needed do let me know!

[alexbakerdev]

Just saw #196 which seems to make this unnecessary, will leave it open in case that PR won't be merged for some time.

[jphines]

@alexbakerdev were you able to verify this functionality in your application?

Per your example,

Cookie Name	Domain
_sso_proxy_staging	staging.example.com
_sso_proxy	example.com

SSO sets cookies per domain (no wildcards) so I would expect the applications on each of these domains not receive the other's _sso_proxy cookie as sent by the browser. This existing application behavior moderately negates the usefulness of this change as described in #197

[alexbakerdev]

Ah yup, as you say by default the cookie is set per domain however we had an interesting use case where we had multiple backend apis that were exposed on different subdomains.
I.e. frontend.domain.com, api.domain.com

Setting the cookie per domain in this case causes issues; since the proxy needs to initiate the authorisation redirect flow for each domain, from client-side at frontend.domain.com making a CORS request to api.domain.com (when you don't already have a valid cookie) the proxy will redirect to the auth server which will fail, as it doesn't support cors.

We solved this by setting the cookie domain to domain.com so the same cookie is used for each of its subdomains, but this caused issues because of the nested nature of the staging sub domain.

I get its a super weird use case, but I feel like there could be other use cases where environments may share a domain for whatever reason, and a similar problem will arise.

EDIT: Also yeah, our fork is solving this problem for us right now.

[jphines]

@alexbakerdev Thanks for the write-up! This makes more sense in given the context!