apps: Support functions components.

[andrewsomething]

This PR adds support functions components to the app resource and data source.

The new acceptance test creates an app with a functions component and then updates it by adding a CORs configuration. The update was triggering an infinite loop as the in-progress deployment completes before we poll the app to find it. I've fixed this with an approach like what we are now doing in doctl (digitalocean/doctl#1143). I've left comments around why this fix is not ideal, but after conversations with the Apps team for the doctl issue, it is currently the best solution. Unfortunately, it actually looks like we missed reviewing a PR attempting to fix this (#818) previously.

$ make testacc TESTARGS='-run=TestAccDigitalOceanApp -count=1'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run=TestAccDigitalOceanApp -count=1 -timeout 120m
?       github.com/digitalocean/terraform-provider-digitalocean [no test files]
=== RUN   TestAccDigitalOceanApp_importBasic
=== PAUSE TestAccDigitalOceanApp_importBasic
=== RUN   TestAccDigitalOceanApp_Image
=== PAUSE TestAccDigitalOceanApp_Image
=== RUN   TestAccDigitalOceanApp_Basic
=== PAUSE TestAccDigitalOceanApp_Basic
=== RUN   TestAccDigitalOceanApp_Job
=== PAUSE TestAccDigitalOceanApp_Job
=== RUN   TestAccDigitalOceanApp_StaticSite
=== PAUSE TestAccDigitalOceanApp_StaticSite
=== RUN   TestAccDigitalOceanApp_InternalPort
=== PAUSE TestAccDigitalOceanApp_InternalPort
=== RUN   TestAccDigitalOceanApp_Envs
=== PAUSE TestAccDigitalOceanApp_Envs
=== RUN   TestAccDigitalOceanApp_Worker
=== PAUSE TestAccDigitalOceanApp_Worker
=== RUN   TestAccDigitalOceanApp_Function
=== PAUSE TestAccDigitalOceanApp_Function
=== RUN   TestAccDigitalOceanApp_Domain
=== PAUSE TestAccDigitalOceanApp_Domain
=== RUN   TestAccDigitalOceanApp_DomainsDeprecation
=== PAUSE TestAccDigitalOceanApp_DomainsDeprecation
=== RUN   TestAccDigitalOceanApp_CORS
=== PAUSE TestAccDigitalOceanApp_CORS
=== RUN   TestAccDigitalOceanApp_TimeoutConfig
=== PAUSE TestAccDigitalOceanApp_TimeoutConfig
=== CONT  TestAccDigitalOceanApp_importBasic
=== CONT  TestAccDigitalOceanApp_Worker
=== CONT  TestAccDigitalOceanApp_TimeoutConfig
=== CONT  TestAccDigitalOceanApp_Domain
=== CONT  TestAccDigitalOceanApp_StaticSite
=== CONT  TestAccDigitalOceanApp_InternalPort
=== CONT  TestAccDigitalOceanApp_DomainsDeprecation
=== CONT  TestAccDigitalOceanApp_Envs
--- PASS: TestAccDigitalOceanApp_TimeoutConfig (33.34s)
=== CONT  TestAccDigitalOceanApp_Function
--- PASS: TestAccDigitalOceanApp_importBasic (146.01s)
=== CONT  TestAccDigitalOceanApp_CORS
--- PASS: TestAccDigitalOceanApp_Domain (152.35s)
=== CONT  TestAccDigitalOceanApp_Basic
--- PASS: TestAccDigitalOceanApp_Function (160.97s)
=== CONT  TestAccDigitalOceanApp_Job
--- PASS: TestAccDigitalOceanApp_DomainsDeprecation (267.98s)
=== CONT  TestAccDigitalOceanApp_Image
--- PASS: TestAccDigitalOceanApp_Image (26.90s)
--- PASS: TestAccDigitalOceanApp_CORS (219.79s)
--- PASS: TestAccDigitalOceanApp_Worker (450.08s)
--- PASS: TestAccDigitalOceanApp_StaticSite (497.34s)
--- PASS: TestAccDigitalOceanApp_InternalPort (595.99s)
--- PASS: TestAccDigitalOceanApp_Job (417.60s)
--- PASS: TestAccDigitalOceanApp_Envs (877.86s)
--- PASS: TestAccDigitalOceanApp_Basic (1010.22s)
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/digitalocean    1162.585s
testing: warning: no tests to run
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/internal/datalist       0.002s [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/internal/mutexkv        0.001s [no tests to run]