provider/fastly: add support for custom VCL configuration (supersedes #6587)

[catsby]

Builds on #6587 by adding an acceptance test, which the original was lacking simply because your account must have custom VCL support enabled

[mattmoyer]

Thanks for finishing this! Tests look good to me.

[catsby]

Tests are passing after a rebase:

TF_ACC=1 go test ./builtin/providers/fastly -v -run=TestAccFastlyServiceV1_ -timeout 120m
=== RUN   TestAccFastlyServiceV1_conditional_basic
--- PASS: TestAccFastlyServiceV1_conditional_basic (14.66s)
=== RUN   TestAccFastlyServiceV1_gzips_basic
--- PASS: TestAccFastlyServiceV1_gzips_basic (28.99s)
=== RUN   TestAccFastlyServiceV1_headers_basic
--- PASS: TestAccFastlyServiceV1_headers_basic (29.51s)
=== RUN   TestAccFastlyServiceV1_s3logging_basic
--- PASS: TestAccFastlyServiceV1_s3logging_basic (28.75s)
=== RUN   TestAccFastlyServiceV1_s3logging_s3_env
--- PASS: TestAccFastlyServiceV1_s3logging_s3_env (12.67s)
=== RUN   TestAccFastlyServiceV1_updateDomain
--- PASS: TestAccFastlyServiceV1_updateDomain (29.71s)
=== RUN   TestAccFastlyServiceV1_updateBackend
--- PASS: TestAccFastlyServiceV1_updateBackend (28.25s)
=== RUN   TestAccFastlyServiceV1_basic
--- PASS: TestAccFastlyServiceV1_basic (12.75s)
=== RUN   TestAccFastlyServiceV1_disappears
--- PASS: TestAccFastlyServiceV1_disappears (7.18s)
=== RUN   TestAccFastlyServiceV1_VCL_basic
--- PASS: TestAccFastlyServiceV1_VCL_basic (28.87s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/fastly 221.365s
Test:

[ghost]

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