feat(rpc): Add client wrapper

[distractedm1nd]

Based on #1175
Closes #1185
Closes #1186

Also adds the basis for the new rpc tests:

1. Making sure all methods we expose over the API actually exist
2. Making sure all the types we return can be marshalled in and out of json
3. RPC specific special behaviors, like authentication

[codecov-commenter]

Codecov Report

Merging #1195 (b7ba257) into main (21aed31) will decrease coverage by 5.20%.
The diff coverage is 64.79%.

@@            Coverage Diff             @@
##             main    #1195      +/-   ##
==========================================
- Coverage   60.25%   55.04%   -5.21%     
==========================================
  Files         161      163       +2     
  Lines        9736     9575     -161     
==========================================
- Hits         5866     5271     -595     
- Misses       3337     3777     +440     
+ Partials      533      527       -6     

Impacted Files	Coverage Δ
api/gateway/availability.go	0.00% <0.00%> (ø)
api/gateway/config.go	0.00% <0.00%> (ø)
api/gateway/das.go	0.00% <ø> (ø)
api/gateway/endpoints.go	0.00% <ø> (ø)
api/gateway/handler.go	0.00% <0.00%> (ø)
api/gateway/header.go	0.00% <ø> (ø)
api/gateway/middleware.go	29.41% <0.00%> (ø)
api/gateway/server.go	68.88% <ø> (ø)
api/gateway/share.go	0.00% <ø> (ø)
api/gateway/state.go	0.00% <ø> (ø)
... and 102 more

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[renaynay]

pushing initial review:
should probs be a api/server and api/client (unless that breaks deps)

[renaynay]

t.Cleanup closer and contexts in tests

[renaynay]

Approving, unit test failure is known flake that will be resolved with #1286