feat: add /showcase/graphql endpoint

[mogery]

Summary:
This PR adds a /showcase/graphql endpoint that behaves like how it's described in #1012. Some caveats:

• For security reasons, the parsed config will not have access to environment variables.
• The Upstream config will be ignored.
• The cache will be shared with the main instance.

This PR also rewrites cloudflare/src/handler.rs to use the new showcase logic. That implementation has none of the above caveats, and should continue to work like it has before, except that it may have even better error reporting.

Issue Reference(s):
Fixes #1012
/claim #1012

Build & Testing:

• I ran cargo test successfully.
• I have run ./lint.sh --mode=fix to fix all linting issues raised by ./lint.sh --mode=check.

Checklist:

• I have added relevant unit & integration tests.
• I have updated the documentation accordingly.
  • feat(docs): add showcase to @server operator tailcallhq.github.io#101
• I have performed a self-review of my code.
• PR follows the naming convention of <type>(<optional scope>): <title>

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (12f4d44) 88.33% compared to head (da582ca) 88.38%.

Files	Patch %	Lines
src/http/showcase.rs	93.05%	5 Missing ⚠️
src/http/request_handler.rs	95.83%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1015      +/-   ##
==========================================
+ Coverage   88.33%   88.38%   +0.04%     
==========================================
  Files         102      103       +1     
  Lines       11034    11115      +81     
==========================================
+ Hits         9747     9824      +77     
- Misses       1287     1291       +4     

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[mogery]

WASM build fails should be fixed. Unfortunately I'm not sure why the linter failing. The action doesn't log any issues and rustfmt is broken for me locally so I can't check for it easily. Might spin up a codespace and try running it there.

[mogery]

Ugh, nevermind, I'm running into compiler problems...

[mogery]

@tusharmath Everything should be resolved now wrt CI checks. Also opened up a codespace to fix lint successfully, my local rustfmt seems to be messed up. What's the codecov success condition?

[mogery]

Rebased on main and changed the internal API a bit. AppContext not having type generics made it quite a bit prettier!

[mogery]

Running into a JS-related http_spec fail on this one. Let's put it on hold 'til I can debug tomorrow.

[mogery]

Alright, this is should be passing everywhere now :)

[mogery]

This is ready now @tusharmath